tbf: Get rid of attribute poll_fn

That field is not needed anymore, and it works only under the assumption
that only 1 poll request can be active at a time per TBF, which is not
true.

Change-Id: I63a34a702f028b871530fb7caeb13e8ea1cc78ac
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 19415f1..c122bba 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -990,11 +990,11 @@
 			rlc.rrbp = rrbp;
 			rlc.es_p = 1; /* Polling */
 
-			m_last_dl_poll_fn = poll_fn;
+			m_last_dl_poll_fn = new_poll_fn;
 
 			LOGPTBFDL(this, LOGL_INFO,
 				  "Scheduled Ack/Nack polling on FN=%d, TS=%d\n",
-				  poll_fn, poll_ts);
+				  new_poll_fn, poll_ts);
 		}
 	}