Delete expired SMS automatically.

Delete expired SMS whenever we are done processing an SMS-related signal.
In order to minimize additional latency only one SMS is removed at a time.

Change-Id: I56cbe716e52b679c4b94f6cbb4a171306975be2e
Related: OS#2354
diff --git a/src/libmsc/sms_queue.c b/src/libmsc/sms_queue.c
index 193d023..163d4c7 100644
--- a/src/libmsc/sms_queue.c
+++ b/src/libmsc/sms_queue.c
@@ -529,6 +529,9 @@
 		     sig_sms->paging_result);
 	}
 
+	/* While here, attempt to remove an expired SMS from the DB. */
+	db_sms_delete_oldest_expired_message();
+
 	return 0;
 }