🐛 polling in registration flow leads to firewall bans on homeservers

### Steps to reproduce

1. Run a firewall on your Homeserver (e.g. using CrowdSec or fail2ban)
2. Open Element Client
3. Set your Homeservers Endpoint
4. Create New Account
5. Enter Registration Token
6. Press Register

### Outcome

#### What did you expect?
The registration to succeed.

#### What happened instead?
You are IP banned from the Homeserver.

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Hello everyone 👋

I discovered a major issue today when using element-web as a client against homeserver instances protected by firewall tooling such as fail2ban or CrowdSec.

While entering the `REGISTRATION_TOKEN`, the app polls the server’s registration endpoint every two seconds, despite the user not having pressed the `Register` button yet. This behavior triggers firewall rules intended to prevent brute-force attacks, causing the user’s IP address to be blocked before any actual registration occurs.

This PR (#31629) removes the unnecessary polling logic, preventing premature IP bans and allowing token-based registration to proceed as expected.

Best regards,
@lukas-runge

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### Operating system

MacOS

### Browser information

Zen

### URL for webapp

/#/register

### Application version

7398a83ae4b4d8f656dea3952e0f72c0cfc3ea6e

### Homeserver

Private Homeserver using Tuwunel

### Will you send logs?

No