Consider using MP_UNREACH_NLRI to withdraw IPv4 Unicast routes after negotiating ENHE

[taspelund]

We currently encode IPv4 Unicast advertisements in MP_REACH_NLRI if we've negotiated ENHE (v6 next-hops), but IPv4 Unicast withdrawals are still encoded via the traditional withdrawn routes field (pre MP-BGP field).

So far I haven't seen interop issues (FRR, Arista EOS) because of this, but we may want to consider encoding v4 uni withdrawals in MP_UNREACH_NLRI when we've negotiated ENHE to avoid possible problems later.