Remove broken Thunderbird protocol selection
This code has been a no-op in practice since at least Tails 4.0. It's misleading and will bitrot.
For context, see #17276 (closed) where we've decided to drop the ball on this, and first investigate what the biggest problems are for email in Tails with slow/shitty Internet connections: default'ing to IMAP may, or may not, be part of these problems.
Edited by intrigeri