Start phasing out 'preferred_mail_format'

Preferred mail format was the idea of sending a message in html or text or both depending what you knew about their preferrences. However, mostly this is an email client setting - you send both and the client deals with it.

We actually invest quite a lot of code & unnecessary queries into this feature & when it came up there was general support for removing it. I also included it in the dev digest on 12 Sep 2021 - so I think we are a couple of emojis away from concept approval on this

https://github.com/civicrm/civicrm-core/pull/21426#pullrequestreview-751423307