This commit removes the TLS cluster join validator.
This validator existed to prevent v6.x nodes (which mandated
TLS) from joining an existing cluster of v5.x nodes (which did
not mandate TLS) unless the 6.x node (and by implication the
5.x nodes) was configured to use TLS.
Since 7.x nodes cannot talk to 5.x nodes, this validator is no longer
needed.
Removing the validator solves a problem where single node clusters
that were bound to local interfaces were incorrectly requiring TLS
when they recovered cluster state and joined their own cluster.
Backport of: #42826
928f49992f