[ruby/rdoc] Remove unused abbrev require

This library originally used `abbrev` to expand abbreviations into
fully-qualified classes, but that was replaced in
f9ffe6684e

`abbrev` is no longer used anywhere, so this commit removes the require.

https://github.com/ruby/rdoc/commit/b76775f27d

lib/rdoc/ri/driver.rb

@@ -1,5 +1,4 @@
 # frozen_string_literal: true
-require 'abbrev'
 require 'optparse'
 
 begin