Don't enqueue preferred body part for fetching twice

For mutipart/alternative mails the code introduced by commit:
a7096490c094608239918ebbdbcca8003e81a8de, enqueues the preferred body part
twice when the content size is bigger that maxMailSize, causing the
part to be written twice.

Signed-off-by: Valerio Valerio <valerio.valerio@jollamobile.com>

Change-Id: I9367494efc19d6a7de73c558044eaab4ca8974c3
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>