Fix AbstractRdapService::remove() deleting the wrong array element

[tomsommer]

AbstractRdapService::remove() looked up the definition in the service's targets list ($service[0]), but then used that offset to unset an element of the service tuple itself ($this->services[$key][$offset]). Removing one target therefore dropped the whole targets list (offset 0) or the URL list (offset 1) of that service:

$service = new DomainService('1.0', 'test', '2024-01-01T00:00:00Z', [
    [['com', 'net'], ['https://rdap.example/com/']],
]);
$service->remove('com');

// before: [[["https://rdap.example/com/"]]]          ('net' lost, URL list moved into the targets slot)
// after:  [[["net"], ["https://rdap.example/com/"]]]

Now the target is removed from the targets list, the service entry is dropped once it has no targets left, and the services list is re-indexed.

phpcs and phpstan pass.