MT#19795 rtc_sessions deprecate network tag

in the future, we will connect one rtc-session to all related
networks (via rtc-accounts). therefore, there is no need anymore,
to store the network tag here. assign a default, in order to ensure
compatibility with old and new code

Change-Id: I76c69250217e1295f511e74a17ef55504e526b80

db_scripts/diff/15248.down

@@ -0,0 +1,7 @@
+USE provisioning;
+SET autocommit=0;
+
+ALTER TABLE rtc_session
+  ALTER COLUMN `rtc_network_tag` DROP DEFAULT;
+
+COMMIT;

db_scripts/diff/15248.up

@@ -0,0 +1,7 @@
+USE provisioning;
+SET autocommit=0;
+
+ALTER TABLE rtc_session
+  ALTER COLUMN `rtc_network_tag` SET DEFAULT '';
+
+COMMIT;