rbinstall.rb: drive letter in without_destdir

* 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

ChangeLog

@@ -1,3 +1,10 @@
+Tue Feb 16 09:51:20 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* 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
+
 Tue Feb 16 04:42:13 2016  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* insns.def (opt_plus): simply use LONG2NUM() instead of wrongly

tool/rbinstall.rb

@@ -282,7 +282,6 @@ end
 
 def without_destdir(dir)
   return dir if !$destdir or $destdir.empty?
-  dir = dir.sub(/\A\w:/, '') if File::PATH_SEPARATOR == ';'
   dir.start_with?($destdir) ? dir[$destdir.size..-1] : dir
 end