Remove RubyGems circural depednecy

[voxik]

I believe that lib/rdoc/rubygems_hook.rb should be removed from RDoc in favor of its counterpart in RubyGems. RDoc does not need RubyGems for anything. OTOH, RubyGems requires RDoc for generating documentation. Therefore, the code belongs to RubyGems IMO, because otherwise there is a dependency loop. Actually, it used to live in RubyGems up until #2235.

TLDR; Please remove lib/rdoc/rubygems_hook.rb in favor of reverting https://github.com/rubygems/rubygems/pull/2235