tbf_dl_ass_fsm: Drop unsued X2000 timer callback

That timer is only relevant for transmission of Packet Access Reject,
which happens only for Uplink assignment, and hence is only set in the
timer of tbf_ul_ass_fsm, never in tbf_dl_ass_fsm. This is probably a
copy-paste artifact when implementing both FSMs.

Change-Id: I95900e211eddb280c72fb712ba1da4d2230cb77b
diff --git a/src/tbf_dl_ass_fsm.c b/src/tbf_dl_ass_fsm.c
index 1d44c91..a5948e7 100644
--- a/src/tbf_dl_ass_fsm.c
+++ b/src/tbf_dl_ass_fsm.c
@@ -185,11 +185,7 @@
 
 static int tbf_dl_ass_fsm_timer_cb(struct osmo_fsm_inst *fi)
 {
-	struct tbf_dl_ass_fsm_ctx *ctx = (struct tbf_dl_ass_fsm_ctx *)fi->priv;
 	switch (fi->T) {
-	case -2000:
-		tbf_free(ctx->tbf);
-		break;
 	default:
 		OSMO_ASSERT(0);
 	}