rubygems/util.rb: remove unused variable

* lib/rubygems/util.rb (Gem::Util.silent_system): remove unused
  variable, to fix test failures with verbose flag.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47046 b2dd03c8-39d4-4d8f-98ff-823fe69b080e