* tool/rbinstall.rb (without_destdir): just strip a drive letter
which is prepended by with_destdir.
pointed out by @DavidEGrayson.
https://github.com/ruby/ruby/commit/0e5f9ae#commitcomment-16101763
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53840 b2dd03c8-39d4-4d8f-98ff-823fe69b080e