Don't run full gc in AfterImportService

Pull mirrors would run the `Projects::AfterImportService`, which would
force a `git gc` each time it finished. This is overkill and not
necessary now that we have refs packed more frequently
(https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27826).

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11556

app/services/projects/after_import_service.rb

@@ -9,7 +9,7 @@ module Projects
     end
     def execute
-      Projects::HousekeepingService.new(@project, :gc).execute do
+      Projects::HousekeepingService.new(@project).execute do
         repository.delete_all_refs_except(RESERVED_REF_PREFIXES)
       end
     rescue Projects::HousekeepingService::LeaseTaken => e

spec/services/projects/after_import_service_spec.rb

@@ -15,7 +15,7 @@ describe Projects::AfterImportService do
   describe '#execute' do
     before do
       allow(Projects::HousekeepingService)
-        .to receive(:new).with(project, :gc).and_return(housekeeping_service)
+        .to receive(:new).with(project).and_return(housekeeping_service)
       allow(housekeeping_service)
         .to receive(:execute).and_yield