`POST /_synapse/mas/delete_user` always triggers deactivations as self-deactivations, despite MAS Admin API being used

[reivilibre]

As at v1.151.0

MasDeleteUserResource has:

        await self.deactivate_account_handler.deactivate_account(
            user_id=user_id.to_string(),
            erase_data=body.erase,
            requester=create_requester(user_id=user_id), # <---
        )

The marked line makes the deactivation trigger as a self-deactivation, even though it could have been requested by an administrator in MAS.

We should also set by_admin properly.