Epics not updated in the index when labels are deleted

Summary

On deleting the label the epics index does not update the label_ids field

Steps to reproduce

1. setup gdk for elasticsearch, enable advanced search, and index everything
2. add a label to an epic
3. delete the label from the group
4. watch elasticsearch.log to see if any epics are queued for indexing. Alrernatively, check on the epics index the label_ids field still contains the deleted label

What is the current bug behavior?

no epics are queued for indexing

What is the expected correct behavior?

epics with the label being deleted are queued for indexing

Relevant logs and/or screenshots

Possible fixes

After fixing the bug don't forget to write a migration to fix the label_ids