Revert "Fix regression spec on case-insensitive fs [skip ci]"

This reverts commit db349bd7aac4d10b43bd3dd1f7873bcdd10e4ffa.
Remove JRUBY-5811 regression spec

Case-insensitive `LOADED_FEATURES` scanning is abandoned (65c07ea).

Fixes #758.
Merge branch 'master' into rubygems-2.0.7