Revert to use Mounter method to check existence

See: https://gitlab.com/gitlab-org/gitlab-ce/commit/6280fd3777920670ee42111fddf29576cbf85988#note_14766241

We wanted to avoid accesses to the file system, because it seems reasonable that if the mounter column has some content it’s because there are an associate file. This is an speed boost when you access to a bunch of build instances to show their information.

The problem is that solution doesn’t work if you uses that method to detect the thing and you’re changing the uploaded file on instances that had that column empty. Until you don’t persist the instance the database column will be empty so we can have false negatives

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