implement removal of MSISDN

Add the first "official" way to remove the MSISDN from a subscriber entry, to
go back to 'MSISDN: none' like just after 'subscriber create'.

Add VTY command 'subscriber <ID> update msisdn none' to drop the MSISDN from
the subscriber. (Like 'subscriber <ID> update aud3g none')

Add DB_STMT_DELETE_MSISDN_BY_IMSI.

In db_subscr_update_msisdn_by_imsi(), allow passing a NULL msisdn, and if NULL,
call above delete SQL statement.

Change-Id: I15419105ea461137776adb92d384d8985210c90e
diff --git a/src/db.h b/src/db.h
index 956b5ed..34582c8 100644
--- a/src/db.h
+++ b/src/db.h
@@ -20,6 +20,7 @@
 	DB_STMT_SUBSCR_CREATE,
 	DB_STMT_DEL_BY_ID,
 	DB_STMT_SET_MSISDN_BY_IMSI,
+	DB_STMT_DELETE_MSISDN_BY_IMSI,
 	DB_STMT_AUC_2G_INSERT,
 	DB_STMT_AUC_2G_DELETE,
 	DB_STMT_AUC_3G_INSERT,