Remove unused let in a provision code suggestions service spec

One of the lets in the specs for a provision code suggestions service
wasn't used after a refactoring anymore. This change removes the unused
let from the spec.

ee/spec/services/gitlab_subscriptions/add_on_purchases/self_managed/provision_code_suggestions_service_spec.rb

@@ -14,7 +14,6 @@
       )
     end
-    let(:expires_at) { Date.current + 6.months }
     let(:purchased_add_on_quantity) { 5 }
     let(:subscription_name) { 'A-S00000002' }