doc/rados: remove dual-stack docs

doc/rados: remove dual-stack docs

Remove references to dual-stack mode in
doc/rados/configuration/network-config-ref.rst and
doc/rados/configuration/msgr2.rst. This feature seems to have been
planned but never to have been completely implemented.

See the tracker issue listed below for an email exchange detailing the
confusion caused by the presence in the documentation of this
now-removed information.

Fixes: https://tracker.ceph.com/issues/65631
Signed-off-by: Zac Dover <zac.dover@proton.me>