PATCH: remove messages from POP cache when deleted

[Sergio Villar Senin <svillar igalia com>]

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Hi,

this is one of the long awaited patches :). With this patch messages
(from a POP account) are removed from disk cache always. The original
code was a bit confusing due two things
   * it never deleted the cached messages because it was trying to
delete an inexistent file
   * in case it ever worked, messages were only deleted from cache if
the message was expunged, something that makes no sense. Messages must
be always removed from cache even though they're not removed from server.

Review it carefully please.

Br