Revert "Fix specs after removing assignee_id field"

This reverts commit 149a535b.

features/steps/groups.rb

@@ -183,7 +183,7 @@ class Spinach::Features::Groups < Spinach::FeatureSteps
   private
 
   def assigned_to_me(key)
-    project.send(key).assigned_to(current_user)
+    project.send(key).where(assignee_id: current_user.id)
   end
 
   def project

spec/services/members/authorized_destroy_service_spec.rb

@@ -24,7 +24,7 @@ describe Members::AuthorizedDestroyService, services: true do
       expect { described_class.new(member, member_user).execute }
         .to change { number_of_assigned_issuables(member_user) }.from(4).to(2)
 
-      expect(issue.reload.assignee_ids).to be_empty
+      expect(issue.reload.assignee_id).to be_nil
       expect(merge_request.reload.assignee_id).to be_nil
     end
   end