Remove support for local messages

Motivation

K-9 Mail currently offers a couple of features that lead to the creation of local messages, i.e. messages that are not connected to a message on the server. However, K-9 Mail doesn't properly support such messages leading to all kinds of problems.

Changing K-9 Mail to properly support local messages requires a lot of work. It would also require a way to import and export local messages so users can change devices and/or clients without losing their messages. While all of these are desirable features I don't see us getting there anytime soon.

Instead I'd like us to concentrate on making K-9 Mail an email client that works great for managing messages stored on a server (the IMAP model). Of course K-9 Mail will still keep local copies of messages and it will still be possible to compose emails while being offline. So we still need limited support for local messages. But those messages will be linked to a message on the server once the app had a chance to upload them. This should require a lot less special cases.

Steps

• #4971 Remove setting 'when I delete a message' (messages deleted locally will always be removed from the server)
• #4973 Remove setting 'sync server deletions' (server deletions will always remove the local copy)
• #4974 Remove POP3 support

Start by reviewing the motivation and the three follow-up issues: #4971, #4973, and #4974. Use those issues to map the settings and POP3 support being removed, while preserving offline composition and limited local copies that become linked to server messages. Done means the listed local-message features are removed without attempting full local-message import or export.