Hosted signaling account can't be deleted after NC hostname changed.

Steps to reproduce

1. Setup Talk on sub1.domain.tld and request a trial of the hosted signaling server.
2. Change the domain of Nextcloud to sub2.otherdomain.tld.
3. Try using Talk.
4. Try to delete the account of the hosted signaling server.

Expected behaviour

• Using Talk (step 3) should fail because the account was requested for a different domain (this is the case). An error message for the user would be nice - currently it just silently doesn't work.
• Deleting the account (step 4) will also fail (same reason), but there should be a way to manually remove the account so a new one can be requested for the new domain.

Actual behaviour

• Talk can no longer be used (step 3), which is OK.
• An error is shown when the trial account is deleted (step 4), but there is no way to manually remove it.

The cause of the failed requests is that for security reasons, the HPB service checks any requests against the hostname an account was registered for.

Start by reproducing the hostname-change scenario in the issue: request a hosted signaling trial, change the Nextcloud hostname, then try Talk and account deletion. Trace the hosted signaling account deletion flow and its hostname validation; done means the old account can be manually removed and the failure to use Talk provides a user-visible error.