Remove duplicated class description

For earlier RubyGems (I guess) the class description was copied directly
into the template.  This is no longer necessary.
Strip links from RDoc from spec reference

RDoc creates cross-reference links that appear in the output but do not
work and create 404s.  This removes those cross-reference links.

This fix is pretty terrible, but there isn't an easy way to remove the
cross-references without writing a lot more code (a custom generator).

Fixes rubygems/rubygems#634.