imap: Removed special EXPUNGE callback for an Outlook workaround.

imap: Removed special EXPUNGE callback for an Outlook workaround.

As the comment mentions, this workaround isn't needed if index files are
used.  Hopefully also the Outlook bug itself is gone nowadays.. In any case
the current callback code didn't work, because it was returning FALSE and
causing an assert in a pipelined EXPUNGE+APPEND combination after 6e9454f:

Panic: file imap-client.c: line 862 (client_check_command_hangs): assertion
failed: (!have_wait_unfinished || unfinished_count > 0)

It would probably be possible to fix this, but it would complicate the code
even further, so it's easier just to get rid of this feature. It was easily
triggered by running imaptest stress testing.