rbinstall.rb: remove unused variable

* tool/rbinstall.rb: $vcs is no longer used since r46513.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47247 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
tool/vcs.rb: no time.rb

* common.mk (Doxyfile): revert r43888, not to require preinstalled
  ruby.  [ruby-core:64488] [Bug #10161]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47248 b2dd03c8-39d4-4d8f-98ff-823fe69b080e