res_pjsip_pubsub: Prevent crashes on final NOTIFY.

The SIP dialog is removed from the subscription tree when the final
NOTIFY is sent. However, after the final NOTIFY is sent, the persistence
update function still attempts to access the cseq from the dialog,
resulting in a crash.

This fix removes the subscription persistence at the same time that the
dialog is removed from the subscription tree. This way, there is no
attempt to update persistence when the subscription is being destroyed.

Change-Id: Ibb46977a6cef9c51dc95f40f43446e3d11eed5bb