BGP: Perform cleanup upon instance delete

BGP: Perform cleanup upon instance delete

When a BGP instance is deleted through 'no router bgp', the required
cleanup was not being performed. This is after VRF-related changes.
Fix to ensure this is taken care of.

Note: Further changes needed in this area for VRFs.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-9597
Reviewed By: CCR-4097
Testing Done: Verified failed test