Add and appy RSpec/RepeatedExampleGroupBody rule

This rule is available since rubocop-rspec 1.38.0. Thanks to it, we've
found out we were testing topics behave like notifiable twice.

spec/models/topic_spec.rb

@@ -68,8 +68,4 @@ describe Topic do
       expect(results).to eq [topic1, topic2, topic3]
     end
   end
-
-  describe "notifications" do
-    it_behaves_like "notifiable"
-  end
 end