Drop support for bind param on POST /account/3pid (MSC2290) (#6067)

As per [MSC2290](https://github.com/matrix-org/matrix-doc/pull/2290/files#diff-05cde9463e9209b701312b3baf2fb2ebR151), we're dropping the bind parameter from `/account/3pid`. This endpoint can now only be used for adding threepid's to the user's account on the homeserver.