[ruby/irb] Remove unnecessary warning check on help command

It's not an intentional behavior of the command and it behaves
differently in different environments. So checking it actually brings
more problem than benefit.

ruby/irb@b3203bc784

test/irb/test_cmd.rb

@@ -422,9 +422,6 @@ def test_help
         irb.eval_input
       end
 
-      # the help command lazily loads rdoc by redefining the execute method
-      assert_match(/discarding old execute/, err) unless RUBY_ENGINE == 'truffleruby'
-
       # the former is what we'd get without document content installed, like on CI
       # the latter is what we may get locally
       possible_rdoc_output = [/Nothing known about String#gsub/, /Returns a copy of self with all occurrences of the given pattern/]
@@ -450,8 +447,6 @@ def test_help_without_rdoc
         end
       end
 
-      # since LoadError will be raised, the execute won't be redefined
-      assert_no_match(/discarding old execute/, err)
       # if it fails to require rdoc, it only returns the command object
       assert_match(/=> IRB::ExtendCommand::Help\n/, out)
     ensure