Remove temporary code, and service references

Remove code marked as temporary support for
https://gitlab.com/gitlab-org/gitlab/-/issues/330300

app/controllers/projects/mattermosts_controller.rb

@@ -41,6 +41,5 @@ class Projects::MattermostsController < Projects::ApplicationController
 
   def integration
     @integration ||= @project.find_or_initialize_integration('mattermost_slash_commands')
-    @service = @integration # TODO: remove when https://gitlab.com/gitlab-org/gitlab/-/issues/330300 is complete
   end
 end

app/controllers/projects/service_hook_logs_controller.rb

@@ -16,6 +16,5 @@ class Projects::ServiceHookLogsController < Projects::HookLogsController
 
   def integration
     @integration ||= @project.find_or_initialize_integration(params[:service_id])
-    @service = @integration # TODO: remove when https://gitlab.com/gitlab-org/gitlab/-/issues/330300 is complete
   end
 end

spec/lib/banzai/filter/references/external_issue_reference_filter_spec.rb

@@ -217,8 +217,6 @@ RSpec.describe Banzai::Filter::References::ExternalIssueReferenceFilter do
   context "ewm project" do
     let_it_be(:integration) { create(:ewm_integration, project: project) }
 
-    let(:service) { integration } # TODO: remove when https://gitlab.com/gitlab-org/gitlab/-/issues/330300 is complete
-
     before do
       project.update!(issues_enabled: false)
     end