cmdmon: remove authentication support

cmdmon: remove authentication support

With the new support for cmdmon over Unix domain sockets, authentication
is no longer necessary to authorize a client running on localhost with
the permissions of the root or chrony user/group. Remove the cmdmon
authentication support to simplify the code and significantly reduce the
attack surface of the protocol.

Only monitoring commands are now allowed remotely. Users that need to
configure chronyd remotely or locally without root/chrony permissions
are advised to use ssh and/or sudo.