rbuninstall.rb: fix for symlink to directory

* tool/rbuninstall.rb: fix for symlink to directory by removing
  trailing slash which let File.symlink?  return false.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39738 b2dd03c8-39d4-4d8f-98ff-823fe69b080e