[ruby/irb] Remove `require` in signal handler to avoid ThreadError

`rdoc` has been required so it looks unnecessary

https://github.com/ruby/irb/commit/5f749c613c

lib/irb/input-method.rb

@@ -319,11 +319,6 @@ module IRB
         [Reline::Key.new(nil, 0xE4, true)], # Normal Alt+d.
         [195, 164] # The "ä" that appears when Alt+d is pressed on xterm.
       ]
-      begin
-        require 'rdoc'
-      rescue LoadError
-        return nil
-      end
 
       if just_cursor_moving and completion_journey_data.nil?
         return nil