vlr: Split vlr_subscr_rx_imsi_detach()

The function vlr_subscr_rx_imsi_detach() implies that an explicit IMSI
DETACH was received.  However, that same function was called in other
situations such as timer expiration or GSUP CANCEL.

Let's clean this up by splitting the function into two parts.

No logical change is introduced to the VLR in this patch.

Change-Id: Iffc02f3062ad591ca372a3c6d866066cf63a8830
1 file changed