3PID removal after losing access to account it was mapped to

**Is your feature request related to a problem? Please describe.**
There is no way to remove 3PIDs if you lose access to the matrix account they are mapped to. If you lose a matrix account with your email mapped to it and create a new matrix account, the email will still point to the old matrix account and there is no way to change that, even with access to the email account.

**Describe the solution you'd like**
A client could make a request to remove a 3PID and go through the same verification steps as making a new 3PID to confirm that the 3PID belongs to the person requesting the removal. The server would then remove the 3PID if the authentication is successful.

**Describe alternatives you've considered**
- a removal request from the 3PID (ie. sending an email or sms message that has a special message requesting to remove the 3PID)

No files, tests, or entry points are named in the issue. Start by reviewing the Matrix specification areas governing 3PID ownership, verification, and removal, then determine the protocol changes needed for a verified removal request. Done means the specification defines the request, authentication flow, and server behavior clearly enough for clients and servers to implement.

Written by the indexing model from the issue text.