BSC_Tests: fix TC_mgwpool_pin_bts: do not leave MGW1 configured

For the purpose of testing MGW pooling, test cases TC_mgwpool_*
configure an additional MGW instance by calling f_vty_mgw_enable()
and then remove it by calling f_vty_mgw_disable().

Unlike the other two TC_mgwpool_* test cases, TC_mgwpool_pin_bts
does not call f_vty_mgw_disable() - this breaks LCLS testcases
executed after it.  Add the missing call.

Change-Id: I3157203888d704b25aabd2569035cd95d48c48b2
Fixes: 3f41e321c74eeab52ad1c8f20b006882e33cf586
Fixes: OS#5727
diff --git a/bsc/BSC_Tests.ttcn b/bsc/BSC_Tests.ttcn
index 0c6efd1..81a1901 100644
--- a/bsc/BSC_Tests.ttcn
+++ b/bsc/BSC_Tests.ttcn
@@ -11954,6 +11954,7 @@
 	vc_conn2.done;
 
 	f_vty_cfg_bts(BSCVTY, 0, { "no mgw pool-target" } );
+	f_vty_mgw_disable(1);
 
 	f_shutdown_helper();
 }