email-clients.txt: sylpheed is OK at IMAP

This comment is not helpful (no reason given) and is incorrect.
Just stick to facts that are useful regarding working on Linux.

(akpm: I've used sylpheed+imap for years)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>