Modernize usage of "restrict" keyword in ntp.conf

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 included some contrib/ntp code inspection to verify that the -4/-6
keywords are basically no-ops in all contexts now.

PR: 201803 210245
Differential Revision: https://reviews.freebsd.org/D15974