Remove unused method respond_to_missing?

NullObject inherits from BasicObject.

BasicObject does not implement respond_to? or respond_to_missing?

Since we're inheriting from BasicObject we don't implement respond_to
and respond_to_missing? in the same way as we would if we were
inheriting from Object

Since respond_to_missing? will never be called, we can delete the unused
code.

In order to stop Rubocop complaining about this we need to disable the
Style/MissingRespondToMissing cop.

References:
https://ruby-doc.org/core-2.7.1/BasicObject.html

lib/factory_bot/null_object.rb

@@ -5,7 +5,7 @@ module FactoryBot
       @methods_to_respond_to = methods_to_respond_to.map(&:to_s)
     end
 
-    def method_missing(name, *args, &block)
+    def method_missing(name, *args, &block) # rubocop:disable Style/MissingRespondToMissing
       if respond_to?(name)
         nil
       else
@@ -16,9 +16,5 @@ module FactoryBot
     def respond_to?(method, _include_private = false)
       @methods_to_respond_to.include? method.to_s
     end
-
-    def respond_to_missing?(*)
-      false
-    end
   end
 end