Remove unused method

In c546a2b0 parameter handling in AC test cases was changed to
round tripping through encoders/decoders so that they matched
reality and in 0adb8f8f the old methods were removed but the
`html_format?` method was overlooked.

actionpack/lib/action_controller/test_case.rb

@@ -668,11 +668,6 @@ def check_required_ivars
             end
           end
         end
-
-        def html_format?(parameters)
-          return true unless parameters.key?(:format)
-          Mime.fetch(parameters[:format]) { Mime["html"] }.html?
-        end
     end
 
     include Behavior