* Enable nullable at the project level
* Remove `#nullable enable` from individual files
Removes `#nullable enable` from almost all .cs files in System.Private.CoreLib. I left it only in the ~30 files (out of ~1480 that had it) that are mirrored to corefx, that are built into projects by corefx that don't yet set NullableContextOptions at the project level, and that use nullable annotations; otherwise, they'd break the corefx build.
Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>
