Completely disable integration manager support

### Steps to reproduce

The docs say: to disable integrations, set the options defined here to null. Introduced with #23489.

1. `integrations_ui_url`: The UI URL for the integration manager.
2. `integrations_rest_url`: The REST interface URL for the integration manager.
3. `integrations_widgets_urls`: A list of URLs the integration manager uses to host widgets.

### Outcome

#### What did you expect?
No way for an user to switch the toggle for the integration manager. Either hide it or grey it out.

#### What happened instead?
The toggle is still there but without function.

### Operating system

Ubuntu

### Application version

Element Version v1.11.10

### How did you install the app?

Docker

### Homeserver

Synapse v1.69.0

### Will you send logs?

No