[ruby/net-http] 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.

https://github.com/ruby/net-http/commit/265bfa929f

lib/net/http.rb

@@ -2573,5 +2573,3 @@ require_relative 'http/response'
 require_relative 'http/responses'
 
 require_relative 'http/proxy_delta'
-
-require_relative 'http/backward'

lib/net/http/backward.rb

@@ -1,40 +0,0 @@
-# frozen_string_literal: true
-# for backward compatibility
-
-# :enddoc:
-
-class Net::HTTP
-  ProxyMod = ProxyDelta
-  deprecate_constant :ProxyMod
-end
-
-module Net::NetPrivate
-  HTTPRequest = ::Net::HTTPRequest
-  deprecate_constant :HTTPRequest
-end
-
-module Net
-  HTTPSession = HTTP
-
-  HTTPInformationCode  = HTTPInformation
-  HTTPSuccessCode      = HTTPSuccess
-  HTTPRedirectionCode  = HTTPRedirection
-  HTTPRetriableCode    = HTTPRedirection
-  HTTPClientErrorCode  = HTTPClientError
-  HTTPFatalErrorCode   = HTTPClientError
-  HTTPServerErrorCode  = HTTPServerError
-  HTTPResponseReceiver = HTTPResponse
-
-  HTTPResponceReceiver = HTTPResponse # Typo since 2001
-
-  deprecate_constant :HTTPSession,
-                     :HTTPInformationCode,
-                     :HTTPSuccessCode,
-                     :HTTPRedirectionCode,
-                     :HTTPRetriableCode,
-                     :HTTPClientErrorCode,
-                     :HTTPFatalErrorCode,
-                     :HTTPServerErrorCode,
-                     :HTTPResponseReceiver,
-                     :HTTPResponceReceiver
-end