Delete SMS from the database once they were sent successfully

Currently the SMS database keeps accumulating entries for each SMS.
These entries are never deleted automatically. With this change, we
start deleting SMS which have successfully been sent to subscriber B.

Change-Id: I3749855fe25d9d4e37ec96b0c2bffbc692b66a78
diff --git a/src/libmsc/sms_queue.c b/src/libmsc/sms_queue.c
index 609a0db..1372a2c 100644
--- a/src/libmsc/sms_queue.c
+++ b/src/libmsc/sms_queue.c
@@ -483,6 +483,7 @@
 		network->sms_queue->pending -= 1;
 		vsub = vlr_subscr_get(pending->vsub);
 		sms_pending_free(pending);
+		db_sms_delete_sent_message_by_id(pending->sms_id);
 		/* Attempt to send another SMS to this subscriber */
 		sms_send_next(vsub);
 		vlr_subscr_put(vsub);