It is no longer necessary to specify a -4/-6 flag on any ntp.conf keyword. The address type is inferred from the address itself as necessary. "restrict default" statements always apply to both address families regardless of any -4/-6 flag that may be present.

So this change just tidies up our default config by removing the redundant restrict -6 statement and comment, and by removing the -6 flag from the restrict keyword that allows access from localhost.

This change was inspired by the patches provided in PRs 201803 and 210245, and was followed up with some contrib/ntp code inspection to verify that the -4/-6 keywords are basically no-ops in all contexts now.