tbf: Kill the tsc member as it duplicates data

We can just use first_ts and the trx/pdch to extract this information.
Avoid duplication of data.
diff --git a/src/bts.cpp b/src/bts.cpp
index 69e5ad4..a25686d 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -450,7 +450,7 @@
 			m_bts.alpha, m_bts.gamma, -1);
 	else
 		plen = Encoding::write_immediate_assignment(&m_bts, immediate_assignment, 0, ra,
-			Fn, tbf->ta, tbf->trx->arfcn, tbf->first_ts, tbf->tsc,
+			Fn, tbf->ta, tbf->trx->arfcn, tbf->first_ts, tbf->tsc(),
 			tbf->tfi(), tbf->dir.ul.usf[tbf->first_ts], 0, 0, 0, 0,
 			m_bts.alpha, m_bts.gamma, -1);
 	pcu_l1if_tx_agch(immediate_assignment, plen);
@@ -506,7 +506,7 @@
 	 * so the assignment will not conflict with possible RACH requests. */
 	plen = Encoding::write_immediate_assignment(&m_bts, immediate_assignment, 1, 125,
 		(tbf->pdch[tbf->first_ts]->last_rts_fn + 21216) % 2715648, tbf->ta,
-		tbf->trx->arfcn, tbf->first_ts, tbf->tsc, tbf->tfi(), 0, tbf->tlli(), poll,
+		tbf->trx->arfcn, tbf->first_ts, tbf->tsc(), tbf->tfi(), 0, tbf->tlli(), poll,
 		tbf->poll_fn, 0, m_bts.alpha, m_bts.gamma, -1);
 	pcu_l1if_tx_pch(immediate_assignment, plen, imsi);
 	bitvec_free(immediate_assignment);