Remove hack for FileUtils.mv due to incorrect error raised from File.rename #5168
In MRI, File.rename will raise an EXDEV if the rename is attempted with paths that span devices (mounts). Up until this PR, we would raise EACCES in such a situation, requiring us to modify fileutils.rb to deal with both errors.
Because this is tweaking a very core behavior of File and FileUtils, I'm pushing to a branch to see how it affects our test runs.
This fix is an attempt to deal with at least part of Bundler's vendoring of a non-JRuby fileutils.rb. See bundler/bundler#6532.