Remove email delegation support

[richvdh]

Back in Synapse 1.4, we gave Synapse the ability to send its own emails for email address validation, but left in the option to delegate this behaviour to an identity server.

Doing delegation in that way is strongly discouraged (for one thing: it allows a malicious/compromised IS to take over HS accounts), and relies on deprecated features of the identity server. I think we should remove this delegation.

[richvdh]

dup #5881