BGP: Unlink BGP instance from VRF only at the end of deletion

BGP: Unlink BGP instance from VRF only at the end of deletion

When a BGP instance including the default instance is deleted, it needs to be
unlinked from the corresponding VRF structure. However, instance deletion does
not happen in one shot but needs a lot of threads to run - peer event handling,
route processing etc. - before it can complete. Premature unlinking of the
instance from underlying VRF would result in BGP routes not being deleted from
the zebra RIB.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-10930
Reviewed By: CCR-4717
Testing Done: Manual, bgp-smoke