bgpd : backpressure - Fix to pop items off zebra_announce FIFO for few EVPN triggers

bgpd : backpressure - Fix to pop items off zebra_announce FIFO for few EVPN triggers

In cases such as 'no advertise-all-vni' and L2 VNI DELETE, we need to
pop all the VPN routes present in the bgp_zebra_announce FIFO yet to
be processed regardless of VNI is configured or not.

NOTE: NO need to pop the VPN routes in two cases
 1) In free_vni_entry
   - Called by bgp_free()->bgp_evpn_cleanup().
   - Since bgp_delete is called before bgp_free and we pop all the dest
     pertaining to bgp under delete.
 2) evpn_delete_vni() when user configures "no vni" since the withdraw
    of all routes happen in normal cycle.

Fixes: a07df6f7548f6bd1b92acbb7a10c3823de33fe5f
("bgpd : backpressure - Handle BGP-Zebra(EPVN) Install evt Creation")

Ticket :#4163611

Signed-off-by: Rajasekar Raja <rajasekarr@nvidia.com>