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.

src/nspawn/nspawn.c

@@ -4636,7 +4636,7 @@ static int run_container(
         if (!barrier_place_and_sync(&barrier)) /* #5 */
                 return log_error_errno(SYNTHETIC_ERRNO(ESRCH), "Child died too early.");
 
-        /* At this point we have made use of the UID we picked, and thus nss-mymachines
+        /* At this point we have made use of the UID we picked, and thus nss-systemd/systemd-machined.service
          * will make them appear in getpwuid(), thus we can release the /etc/passwd lock. */
         etc_passwd_lock = safe_close(etc_passwd_lock);