BSC_Tests_VAMOS: fix f_TC_mode_modify_to_vamos(): clean up lchan

This change makes the following test cases pass:

  * TC_mode_modify_to_vamos_fr
  * TC_mode_modify_to_vamos_hr

Change-Id: Icb47e39a5466a1232cda03e1e04b0c88f3927659
Related: OS#5444
diff --git a/bsc/BSC_Tests_VAMOS.ttcn b/bsc/BSC_Tests_VAMOS.ttcn
index 99fe92d..ca722e7 100644
--- a/bsc/BSC_Tests_VAMOS.ttcn
+++ b/bsc/BSC_Tests_VAMOS.ttcn
@@ -350,6 +350,7 @@
 
 private function f_TC_mode_modify_to_vamos(charstring id) runs on MSC_ConnHdlr {
 	f_est_lchan_and_mode_modify_to_vamos();
+	f_perform_clear(RSL);
 }
 
 /* Modify a primary lchan into VAMOS speech mode */