bpf: Drop mpls from bpf_fib_lookup

MPLS support will not be submitted this dev cycle, but in working on it
I do see a few changes are needed to the API. For now, drop mpls from the
API. Since the fields in question are unions, the mpls fields can be added
back later without affecting the uapi.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>