tbf: Get rid of attribute poll_ts

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: I9b8bed7741d385bab4cd8c64b841a78a02a05fe1
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index c122bba..dd5d0f2 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -994,7 +994,7 @@
 
 			LOGPTBFDL(this, LOGL_INFO,
 				  "Scheduled Ack/Nack polling on FN=%d, TS=%d\n",
-				  new_poll_fn, poll_ts);
+				  new_poll_fn, ts);
 		}
 	}