bgpd: clear l3vni prefix-only flag upon deletion

bgpd: clear l3vni prefix-only flag upon deletion

When L3vni is created with prefix-only flag,
the flag is set at bgp vrf instance level.
In the case of bgp instance is non auto created,
means user configured instance (i.e 'router bgp x vrf <name>')

Upon deletion of l3vni, clear the prefix-only flag from
bgp vrf instance.

Ticket:CM-21894
Reviewed By:CCR-9176
Testing Done:

vrf vrf1
 vni 104001
 exit-vrf
!
router bgp 650030 vrf vrf1
!

tor-21(config)# vrf vrf1
tor-21(config-vrf)# vni 104001 prefix-routes-only
tor-21(config-vrf)# no vni 104001 prefix-routes-only
tor-21(config-vrf)# end

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>