tevent: Only set poll_ev->delete=false if it was true

Might not be noticable, but I thought it would be an obvious tiny
optimization. Possibly the compiler already does this.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>

Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Thu Apr  4 18:32:39 CEST 2013 on sn-devel-104