Fix TSC/BSIC handling bug and remove bts->tsc

This fixes a bug in the following circumstances:
* BSIC is set to 0 in the config file
* No TSC is explicitly specified at the BST level in the config file

In this case, we ended up using BSIC=0 and TSC=7, as TSC=7 is our
default initialization value.

The TSC of the CCCH/BCCH must always be the BCC, which is the lower 3
bits of the BSIC.  Having configuration options for both the BSIC _and_
the TSC at the BTS level therefore makes no sense, as it only adds ways
in which users can configure non-oprational configurations.  So we
remove the bts->tsc member, and keep only the ts->tsc members that allow
us to configure a timeslot-specific TSC that's different from the BTS
TSC (= BCC).
diff --git a/openbsc/src/libcommon/gsm_data.c b/openbsc/src/libcommon/gsm_data.c
index 4ce4eca..16035ed 100644
--- a/openbsc/src/libcommon/gsm_data.c
+++ b/openbsc/src/libcommon/gsm_data.c
@@ -294,7 +294,7 @@
 }
 
 struct gsm_bts *gsm_bts_alloc_register(struct gsm_network *net, enum gsm_bts_type type,
-					uint8_t tsc, uint8_t bsic)
+					uint8_t bsic)
 {
 	struct gsm_bts_model *model = bts_model_find(type);
 	struct gsm_bts *bts;
@@ -310,7 +310,6 @@
 	bts->nr = net->num_bts++;
 	bts->type = type;
 	bts->model = model;
-	bts->tsc = tsc;
 	bts->bsic = bsic;
 
 	bts->neigh_list_manual_mode = 0;