sgsn: Replace subscr.authenticate by global require_authentication flag

Currently the flag 'authenticate' is managed per subscriber.

This patch replaces that flag by a global cfg.require_authentication
flag that enables/disables the use of the Auth & Ciph procedure for
every subscriber. The flag is set by the VTY, if and only if the
authorization policy is 'remote'.

The VTY command

  - update-subscriber imsi IMSI insert authenticate <0-1>

is removed.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/sgsn_test.c b/openbsc/tests/sgsn/sgsn_test.c
index ebe12c9..7c48eef 100644
--- a/openbsc/tests/sgsn/sgsn_test.c
+++ b/openbsc/tests/sgsn/sgsn_test.c
@@ -876,7 +876,7 @@
 
 	subscr = gprs_subscr_get_or_create("123456789012345");
 	subscr->authorized = 1;
-	subscr->sgsn_data->authenticate = 1;
+	sgsn->cfg.require_authentication = 1;
 	subscr_put(subscr);
 
 	printf("Auth policy 'remote', auth faked: ");
@@ -918,7 +918,7 @@
 
 	subscr = gprs_subscr_get_or_create("123456789012345");
 	subscr->authorized = 1;
-	subscr->sgsn_data->authenticate = 1;
+	sgsn->cfg.require_authentication = 1;
 	subscr_put(subscr);
 
 	printf("Auth policy 'remote', triplet based auth: ");