Fix rubocop-rspec by removing a redundant let

Only allowed 15 `let` or `subject`s for a given spec, and we now have 16.
Removing the `with_null` memoization lets us proceed. In theory it would have
allowed us to run the specs with_null set to both true and false, but in
practice, we weren't, and the other memoized values _were_ useful for
customising the specs.

spec/lib/apipie/generator/swagger/param_description/type_spec.rb

@@ -3,7 +3,6 @@
 describe Apipie::Generator::Swagger::ParamDescription::Type do
   let(:validator_options) { {} }
   let(:param_description_options) { {}.merge(validator_options) }
-  let(:with_null) { false }
   let(:http_method) { :GET }
   let(:path) { '/api' }
   let(:validator) { String }
@@ -67,7 +66,7 @@
 
   let(:type_definition) do
     described_class.
-      new(param_description, with_null: with_null, controller_method: controller_method).
+      new(param_description, with_null: false, controller_method: controller_method).
       to_hash
   end