[ruby/irb] Drop hard dependency on RDoc.

This has been introduced in https://github.com/ruby/irb/commit/026700499dfd,
but it seems that this is just be mistake, otherwise the later handling
of `LoadError` would not be needed.

https://github.com/ruby/irb/commit/54c8df06ff

lib/irb/input-method.rb

@@ -14,7 +14,6 @@ require_relative 'magic-file'
 require_relative 'completion'
 require 'io/console'
 require 'reline'
-require 'rdoc'
 
 module IRB
   STDIN_FILE_NAME = "(line)" # :nodoc: