This was recommended by @t-bast: if the final spec commits to something
compatible, we can simply advertize and accept both features, but if it
does change in incompatible ways we won't cause problems for nodes
who implement the official spec.
(I split this, so first, we remove the OPT_SPLICE entirely, to make
sure we caught them all. --RR)
Suggested-by: @t-bast
Changelog-None