No option to remove logos in the admin panel

In the admin panel you can add an image for the website logo, but there isn't an option to remove it. This was flagged as an issue by a community member who is working on creating a prototype for a corporate client, they were able to replace the logo with another image, but they were concerned that its possible a corporate client would ask for the branding to be removed from a site entirely.

This has also been flagged by other Hubs Cloud users.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/HUBS-1049)

Start by locating the admin panel's website-logo settings and tracing how an uploaded logo is stored and displayed. Verify the existing replacement flow, then define the removal behavior and check that a configured logo can be cleared so the site displays no branding image.

Written by the indexing model from the issue text.