tbf: Remove IMSI handling from trigger_dl_ass

Currently the BTS::trigger_dl_ass() method assigns the IMSI to the MS
object. This should be (and is already) done earlier where the MS
object is retrieved/created.

This commit removes the corresponding code along with the 'imsi'
parameter from trigger_dl_ass.

Sponsored-by: On-Waves ehf
diff --git a/src/bts.h b/src/bts.h
index e5733e4..2e2d146 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -216,7 +216,7 @@
 	int rcv_imm_ass_cnf(const uint8_t *data, uint32_t fn);
 	int rcv_rach(uint8_t ra, uint32_t Fn, int16_t qta);
 
-	void trigger_dl_ass(gprs_rlcmac_dl_tbf *tbf, gprs_rlcmac_tbf *old_tbf, const char *imsi);
+	void trigger_dl_ass(gprs_rlcmac_dl_tbf *tbf, gprs_rlcmac_tbf *old_tbf);
 	void snd_dl_ass(gprs_rlcmac_tbf *tbf, uint8_t poll, const char *imsi);
 
 	GprsMsStorage &ms_store();