ci: Fix Danger job being broken because of duplicate Gemfiles

Recently our Danger job has started failing with the following error
message:

```
[!] There was an error parsing `injected gems`: You cannot specify the same gem twice with different version requirements.
You specified: gitlab-dangerfiles (~> 3.1.0) and gitlab-dangerfiles (>= 0). Gem already added. Bundler cannot continue.
```

As it turns out, this breakage is caused by the upstream change 9e3eff4
(Don't require projects to contain Gemfile when using danger-review job,
2022-06-10) in the common pipeline configurations: if the CI job detects
that there is no Gemfile, it has now started to write one for us that
contains the `gitlab-dangerfiles` version. And because our Gemfile is
not contained in the root directory but instead in the `danger/` sub
directory we now end up with two Gemfiles.

Fix this issue by making use of the new feature: instead of carrying our
own Gemfile, we can now simply rely on the auto-generated Gemfile. While
this forces us to get rid of the Ruby cache which was keyed by our own
Gemfile, this is really not an issue at all: execution of the whole job
only takes about 40 seconds without the cache, which is fast enough.

3 files changed, 1 insertions, 113 deletions

diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 916e6dfae..d969f49a7 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -120,15 +120,7 @@ danger-review:
   stage: build
   allow_failure: true
   variables:
-    BUNDLE_GEMFILE: danger/Gemfile
-  cache:
-    key:
-      files:
-        - danger/Gemfile.lock
-      prefix: debian-${DEBIAN_VERSION}-ruby-${RUBY_VERSION}
-    paths:
-      - .ruby
-    policy: pull-push
+    GITLAB_DANGERFILES_VERSION: "3.1.0"
 
 build:
   needs: []
diff --git a/danger/Gemfile b/danger/Gemfile
deleted file mode 100644
index 0d89eafbc..000000000
--- a/danger/Gemfile
+++ /dev/null
@@ -1,3 +0,0 @@
-source 'https://rubygems.org'
-
-gem 'gitlab-dangerfiles', '~> 3.1.0', require: false
diff --git a/danger/Gemfile.lock b/danger/Gemfile.lock
deleted file mode 100644
index 6dc374878..000000000
--- a/danger/Gemfile.lock
+++ /dev/null
@@ -1,101 +0,0 @@
-GEM
-  remote: https://rubygems.org/
-  specs:
-    addressable (2.8.0)
-      public_suffix (>= 2.0.2, < 5.0)
-    claide (1.1.0)
-    claide-plugins (0.9.2)
-      cork
-      nap
-      open4 (~> 1.3)
-    colored2 (3.1.2)
-    cork (0.3.0)
-      colored2 (~> 3.1)
-    danger (8.6.0)
-      claide (~> 1.0)
-      claide-plugins (>= 0.9.2)
-      colored2 (~> 3.1)
-      cork (~> 0.1)
-      faraday (>= 0.9.0, < 2.0)
-      faraday-http-cache (~> 2.0)
-      git (~> 1.7)
-      kramdown (~> 2.3)
-      kramdown-parser-gfm (~> 1.0)
-      no_proxy_fix
-      octokit (~> 4.7)
-      terminal-table (>= 1, < 4)
-    danger-gitlab (8.0.0)
-      danger
-      gitlab (~> 4.2, >= 4.2.0)
-    faraday (1.10.0)
-      faraday-em_http (~> 1.0)
-      faraday-em_synchrony (~> 1.0)
-      faraday-excon (~> 1.1)
-      faraday-httpclient (~> 1.0)
-      faraday-multipart (~> 1.0)
-      faraday-net_http (~> 1.0)
-      faraday-net_http_persistent (~> 1.0)
-      faraday-patron (~> 1.0)
-      faraday-rack (~> 1.0)
-      faraday-retry (~> 1.0)
-      ruby2_keywords (>= 0.0.4)
-    faraday-em_http (1.0.0)
-    faraday-em_synchrony (1.0.0)
-    faraday-excon (1.1.0)
-    faraday-http-cache (2.2.0)
-      faraday (>= 0.8)
-    faraday-httpclient (1.0.1)
-    faraday-multipart (1.0.3)
-      multipart-post (>= 1.2, < 3)
-    faraday-net_http (1.0.1)
-    faraday-net_http_persistent (1.2.0)
-    faraday-patron (1.0.0)
-    faraday-rack (1.0.0)
-    faraday-retry (1.0.3)
-    git (1.11.0)
-      rchardet (~> 1.8)
-    gitlab (4.18.0)
-      httparty (~> 0.18)
-      terminal-table (>= 1.5.1)
-    gitlab-dangerfiles (3.1.0)
-      danger (>= 8.4.5)
-      danger-gitlab (>= 8.0.0)
-      rake
-    httparty (0.20.0)
-      mime-types (~> 3.0)
-      multi_xml (>= 0.5.2)
-    kramdown (2.3.2)
-      rexml
-    kramdown-parser-gfm (1.1.0)
-      kramdown (~> 2.0)
-    mime-types (3.4.1)
-      mime-types-data (~> 3.2015)
-    mime-types-data (3.2022.0105)
-    multi_xml (0.6.0)
-    multipart-post (2.1.1)
-    nap (1.1.0)
-    no_proxy_fix (0.1.2)
-    octokit (4.22.0)
-      faraday (>= 0.9)
-      sawyer (~> 0.8.0, >= 0.5.3)
-    open4 (1.3.4)
-    public_suffix (4.0.7)
-    rake (13.0.6)
-    rchardet (1.8.0)
-    rexml (3.2.5)
-    ruby2_keywords (0.0.5)
-    sawyer (0.8.2)
-      addressable (>= 2.3.5)
-      faraday (> 0.8, < 2.0)
-    terminal-table (3.0.2)
-      unicode-display_width (>= 1.1.1, < 3)
-    unicode-display_width (2.1.0)
-
-PLATFORMS
-  ruby
-
-DEPENDENCIES
-  gitlab-dangerfiles (~> 3.1.0)
-
-BUNDLED WITH
-   2.2.33