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