clean-up-last-modifier-id: Don't delete guest users

Fixes sqlalchemy.exc.IntegrityError: (raised as a result of Query-invoked autoflush; consider using a session.no_autoflush block if this flush is occurring prematurely)
(psycopg2.errors.ForeignKeyViolation) update or delete on table "account" violates foreign key constraint "session_last_modifier_id_fkey" on table "session"
DETAIL:  Key (id)=(...) is still referenced from table "session".

Ref syslabcom/scrum#2372

src/euphorie/deployment/browser/maintenance.py

@@ -2,6 +2,7 @@
 from euphorie.client.model import Session
 from euphorie.client.model import SurveySession
 from Products.Five import BrowserView
+from sqlalchemy.exc import SQLAlchemyError
 
 import logging
 
@@ -29,7 +30,4 @@ def __call__(self):
                 .filter(SurveySession.account_id == guest_user.id)
                 .count()
             )
-            if num_assessments == 0:
-                session.delete(guest_user)
-                logger.info("Deleted user %s", guest_user.id)
         return "Done"