Remove deprecated constants

These constants, isolated in net/http/backward.rb, have not only been
deprecated since 2001, but have also had a warning since 2021.

Author: nobu

lib/net/http.rb

@@ -2573,5 +2573,3 @@ def debug(msg)
 require_relative 'http/responses'
 
 require_relative 'http/proxy_delta'
-
-require_relative 'http/backward'