unifdef TLS13_USE_LEGACY_CLIENT_AUTH

unifdef TLS13_USE_LEGACY_CLIENT_AUTH

Before the TLSv1.3 stack grew client certificate support, it fell back
to the legacy stack. Proper client certificate support was added in a2k20
with a TLS13_USE_LEGACY_CLIENT_AUTH knob to provide an easy fallback in
case the new code should have a problem. This was never needed.

As ifdefed code is wont to do, this bitrotted a few months later when
the client and server methods were merged.

discussed with jsing