PCUIF_Codec port, do not return fn in f_PCUIF_tx_imm_ass_pch

The function f_PCUIF_tx_imm_ass_pch() retuns the frame number, that is
sent back from osmo-bts via PCUIF / gsm_pcu_if_data_cnf_dt. Since we are
about to remove this field from gsm_pcu_if_data_cnf_dt, lets no longer
access it here as well. Also the return code is never used anywhere in
the tests. (In osmo-pcu the fn parameter was used for logging only, in
osmo-bts it was set to constant 0)

Related: OS#5927
Change-Id: I0e5bad7a0d74e5032f2818f6fdf5b9b2ecb531cc
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index e8ae1c0..ad6ae5a 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -5848,7 +5848,7 @@
 	f_l1_tune(L1CTL);
 
 	/* append 3 last imsi digits so BTS can compute pagng group */
-	var uint32_t fn := f_PCUIF_tx_imm_ass_pch(PCU, g_pcu_conn_id, imm_ass, '123459987'H);
+	f_PCUIF_tx_imm_ass_pch(PCU, g_pcu_conn_id, imm_ass, '123459987'H);
 
 	timer T := 0.5;
 	T.start;