bgpd: move router bgp nb callback

bgpd: move router bgp nb callback

move `router bgp` nb callback at `bgp` node level
to have access to bgp context at neighbor and peer-group
level and align create/destroy callbacks call during
no router bgp.

Earlier `no router bgp` is performed first global destroy
callback is called which essentially removes `bgp context`
then it calls to remove (parallel nodes) neighbor and peer-group
which does not have access to bgp context.
Moving router bgp at bgp solves this destroy callback ordering issue.

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