core: drop implicit support of PrivateUsers=off

Follow-up for fa693fdc7e.

The documentation says the option takes a boolean or one of the "self"
and "identity". But the parser uses private_users_from_string() which
also accepts "off". Let's drop the implicit support of "off".

src/core/dbus-execute.c

@@ -1038,7 +1038,7 @@ static int property_get_private_users(
                 sd_bus_error *error) {
 
         PrivateUsers *p = ASSERT_PTR(userdata);
-        int b = *p != PRIVATE_USERS_OFF;
+        int b = *p != PRIVATE_USERS_NO;
 
         return sd_bus_message_append_basic(reply, 'b', &b);
 }
@@ -1882,7 +1882,7 @@ int bus_exec_context_set_transient_property(
                         return r;
 
                 if (!UNIT_WRITE_FLAGS_NOOP(flags)) {
-                        c->private_users = v ? PRIVATE_USERS_SELF : PRIVATE_USERS_OFF;
+                        c->private_users = v ? PRIVATE_USERS_SELF : PRIVATE_USERS_NO;
                         (void) unit_write_settingf(u, flags, name, "%s=%s", name, yes_no(v));
                 }