bgpd: upon bgp deletion, do not systematically ask to remove main bgp

bgpd: upon bgp deletion, do not systematically ask to remove main bgp

Dependencies between bgp instances is necessary only when it comes to
configure some specific services like ipv4-vpn, ipv6-vpn or l2vpn-evpn.
The list of config possibilities is listed, and an error is returned if
one of the above services is configured on the bgp vrf instance.

There may be some missingn services not covered. For clarification, here
are services configured on bgp vrf instances, while trying to delete
main bgp instance:
- if evpn main instance is the main bgp instance, and if evpn rt5
service is configured (with advertise command)
- if a vni is configured in the vrf instance
- if l3vpn import/export commands are solicitated for
importing/exporting entries from a vpnv4/6 network located on main bgp
instance. (in l3vpn, the main bgp instance is the location where vpnv4/6
sits).

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>