lib: fix false context information for SRv6 route

lib: fix false context information for SRv6 route

The seg6local route dumped by 'show ipv6 route' makes think that the USP
flavor is supported, whereas it is not the case. This information is a
context information, and for End, the context information should be
empty.

> # show ipv6 route
> [..]
> I>* fc00:0:4::/128 [115/0] is directly connected, sr0, seg6local End USP, weight 1, 00:49:01

Fix this by suppressing the USP information from the output.

Fixes: e496b4203055 ("bgpd: prefix-sid srv6 l3vpn service tlv")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>