Avoid arity check in Mock#respond_to_missing?

As far as I can see, Object#respond_to? has supported an optional 2nd
argument since at least Ruby v1.8.6 and perhaps forever. The only thing
thing that changed was that in Ruby v2 onwards the optional 2nd argument
changed its name from include_private to include_all. I'm not sure
whether this was just a clarification about the behaviour for protected
methods or whether prior to v2 protected methods were never checked.

In any case, it seems to make sense to assume that a respond_to? method
always supports an optional 2nd argument and avoid the arity check in
Mock#respond_to_missing?. My suspicion is that this check was only added
to avoid having to fix some tests in MockTest which I've since resolved
in an earlier commit.

lib/mocha/mock.rb

@@ -324,11 +324,7 @@ def method_missing(symbol, *arguments, &block) # rubocop:disable Style/MethodMis
     # @private
     def respond_to_missing?(symbol, include_private = false)
       if @responder
-        if @responder.method(:respond_to?).arity > 1
-          @responder.respond_to?(symbol, include_private)
-        else
-          @responder.respond_to?(symbol)
-        end
+        @responder.respond_to?(symbol, include_private)
       else
         @everything_stubbed || all_expectations.matches_method?(symbol)
       end