nss-mymachines: drop support for UID/GID resolving

      Now that we make the user/group name resolving available via userdb and
thus nss-systemd, we do not need the UID/GID resolving support in
nss-mymachines anymore. Let's drop it hence.

We keep the module around, since besides UID/GID resolving it also does
hostname resolving, which we care about. (One of those days we should
replace that by some Varlink logic between
nss-resolve/systemd-resolved.service too)

The hooks are kept in the NSS module, but they do not resolve anything
anymore, in order to keep compat at a maximum.