Description of changes:
In firebase_admin/_user_mgt.py we changed the email validation in the payload to include the condition that checks whether email should be deleted via DELETE_ATTRIBUTE. Also we wrote the function to delete email from the database upon checking DELETE_ATTRIBUTE.

Description of tests:
In tests/test_user_mgt.py we included corresponding fields for email in the function test_update_user_delete_fields. Then we wrote the function test_update_user_delete_provider_and_email that verifies email deletion from the database in the same manner as phone number deletion.

cc @renkelvin