detach cancelled subscribers from VLR

When a subscriber is cancelled, fake an IMSI detach to
ensure that the subscriber gets removed from the VLR.

I am not entirely sure if this change is correct but
it does make TTCN3 test MSC_Tests.TC_gsup_cancel pass.

Change-Id: I5918106e4a94ba2e6c61bcd7b90d3bf0565513cc
Related: OS#2886

src/libvlr/vlr.c

@@ -970,6 +970,8 @@ static int vlr_subscr_handle_cancel_req(struct vlr_subscr *vsub,
 	gmm_cause_to_fsm_and_mm_cause(gsup_msg->cause, &fsm_cause, &gsm48_rej);
 	vlr_subscr_cancel_attach_fsm(vsub, fsm_cause, gsm48_rej);
 
+	vlr_subscr_rx_imsi_detach(vsub);
+
 	return rc;
 }