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/tbf.cpp b/src/tbf.cpp
index 0771969..4bafe14 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -621,7 +621,7 @@
 
 				dl_tbf->update();
 
-				dl_tbf->bts->trigger_dl_ass(dl_tbf, dl_tbf, NULL);
+				dl_tbf->bts->trigger_dl_ass(dl_tbf, dl_tbf);
 			} else
 				LOGP(DRLCMAC, LOGL_NOTICE, "%s Continue flow after "
 					"IMM.ASS confirm\n", tbf_name(dl_tbf));