bgpd: do not display vty output headers for link-state prefixes

bgpd: do not display vty output headers for link-state prefixes

When displaying the link-state prefixes with "show bgp link-state
link-state" command, the following output headers are not needed:

> Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
> 	       i internal, r RIB-failure, S Stale, R Removed
> Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
> Origin codes:  i - IGP, e - EGP, ? - incomplete
> RPKI validation codes: V valid, I invalid, N Not found

Do not display these headers for link-state SAFI.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>