Delete the old localized outbox. Fixes the "two outboxes" problem.

Rename the "new" hardcoded outbox to "K9MAIL_INTERNAL_OUTBOX". This
is done to avoid collisions with folders named "OUTBOX" in a remote
store. See
https://groups.google.com/group/k-9-mail/browse_thread/thread/cbb1c77abba84695

Fixes issue 3411