Stop using /_matrix/identity/api/v1/3pid/unbind

[richvdh]

/_matrix/identity/api/v1/3pid/unbind was removed from the spec over a year ago (msc2713); we should no longer be using it.

For context, it is called from IdentityHandler.try_unbind_threepid_with_id_server, whenever a user requests a threepid be unbound or their HS account be deleted.

Since the IS accepts requests without an id server access token to /_matrix/identity/v2/3pid/unbind, the solution here is just to switch to that.

(The spec requires that the IS accept requests requests signed by the homeserver which controls the user_id).

[richvdh]

This is part of #9677.