[subscr] Remove the struct gsm_bts pointer from subscriber

The pointer was initially added to support to open a lchan
by using the gsm_subscriber structure. We would have restored
the struct gsm_bts from the db when loading the subscriber.

Instead we will dynamically resolve the gsm_bts and might end
up paging multiple bts in the same (stored/old) location area
until it answers.
diff --git a/src/gsm_subscriber.c b/src/gsm_subscriber.c
index 4888506..3f608ec 100644
--- a/src/gsm_subscriber.c
+++ b/src/gsm_subscriber.c
@@ -96,17 +96,13 @@
 	/* FIXME: Migrate pending requests from one BSC to another */
 	switch (reason) {
 	case GSM_SUBSCRIBER_UPDATE_ATTACHED:
-		s->current_bts = bts;
 		/* Indicate "attached to LAC" */
 		s->lac = bts->location_area_code;
 		break;
 	case GSM_SUBSCRIBER_UPDATE_DETACHED:
-		/* Only detach if we are currently attached to this bts */
-		if (bts == s->current_bts) {
-			s->current_bts = NULL;
-			/* Indicate "detached" */
+		/* Only detach if we are currently in this area */
+		if (bts->location_area_code == s->lac)
 			s->lac = 0;
-		}
 
 		break;
 	default: