`'forward', <reason>` hook is triggered into the `forward()` function, passing as `<reason>` it's second parameter (usually 'system' or '404'). The only way we have to distinguish if we are in a register or login forward is its `current_url` param (matching it with register or login action url).

`login:forward` hook is different. It is triggered **outside** forward, from `actions/login.php`, and is getting some parameters that can't be passed to `forward, <reason>` hook (except if we add a third parameter to `forward()` function, to extend the `forward` hook parameters, and that's not a bad idea).

I can use `forward` hook in this case, so i remove `register:forward` hook. We can consider in another issue adding hook params as a third parameter in `forward()` function and deprecating `login:forward` hook.