Remove `account_threepid_delegates` option

This issue has been migrated from [#5881](https://github.com/matrix-org/synapse/issues/5881).

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With https://github.com/matrix-org/synapse/pull/5835, we're allowing homeservers to be able to send their own registration emails. Earlier, we allowed homeservers to be able to send password reset emails themselves. Both of these options were just considered temporary to migrate people over from relying on vector.im or another identity server to handle SMTP, to handling SMTP themselves if they required the functionality.

Once a sufficient amount of time has passed, we should remove these options and yell from the mountains that people need to manage SMTP from their Synapse install or disable it altogether.