[hooks] remove buggy and unused import of FormatConstraint

hooks.py

@@ -15,7 +15,6 @@ import re
 from logilab.mtconverter import TransformError
 
 from cubicweb import UnknownEid
-from cubicweb.schema import FormatConstraint
 from cubicweb.server.pool import PreCommitOperation
 from cubicweb.server.hooksmanager import Hook
 from cubicweb.sobjects.notification import parse_message_id