Drop support for delegating email validation (#13192)

* Drop support for delegating email validation

Delegating email validation to an IS is insecure (since it allows the owner of
the IS to do a password reset on your HS), and has long been deprecated. It
will now cause a config error at startup.

* Update unit test which checks for email verification

Give it an `email` config instead of a threepid delegate

* Remove unused method `requestEmailToken`

* Simplify config handling for email verification

Rather than an enum and a boolean, all we need here is a single bool, which
says whether we are or are not doing email verification.

* update docs

* changelog

* upgrade.md: fix typo

* update version number

this will be in 1.64, not 1.63

* update version number

this one too