delete deferred events after they're executed
(otherwise they can be executed again for a sub-statement)
See also
commit 0e78d1d
Author: Venkatesh Duggirala <venkatesh.duggirala@oracle.com>
Date: Wed Mar 20 11:20:47 2013 +0530
BUG#15850951-DUPLICATE ERROR IN REPLICATION WITH SLAVE
TRIGGERS AND AUTO INCREMENT