Revert "dir.c: glob cases on case-insensitive system"

This reverts commit r44796.

    * dir.c (glob_helper): return the filename with actual cases on
      the filesystem if it is case-insensitive.  [ruby-core:42469]
      [Feature #5994]

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44801 b2dd03c8-39d4-4d8f-98ff-823fe69b080e