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/include/osmocom/msc/db.h b/include/osmocom/msc/db.h
index 988c9bd..13c5ed3 100644
--- a/include/osmocom/msc/db.h
+++ b/include/osmocom/msc/db.h
@@ -49,6 +49,7 @@
 int db_sms_mark_delivered(struct gsm_sms *sms);
 int db_sms_inc_deliver_attempts(struct gsm_sms *sms);
 int db_sms_delete_by_msisdn(const char *msisdn);
+int db_sms_delete_sent_message_by_id(unsigned long long sms_id);
 
 /* Statistics counter storage */
 struct osmo_counter;