Remove just deleted POP3 messages from the internal maps

This is to prevent attempts to delete them again.

Fixes the testCountBasedLeaveRule, which excercises the following
scenario:

 * initial state: 3 messages known
 * two new messages are added on server
 * LIST returns 5 messages
 * UIDL returns 5 UIDs, 2 of them new
 * download of 2 messages starts
 * fetch of first message is completed
 * item create for first message is completed
 * item create completion triggers cleanup of unwanted server messages
 * messages 1 and 3 are deleted as they are the oldest
 * fetch of second message is completed
 * item create for second message is completed
 * item create completion triggers cleanup of unwanted server messages
 * messages 1 and 3 are deleted again - this causes the test to fail

Removing those identifiers causes the second search for items to
delete to return an empty set and no further delete attempts are made.

Reviewers: #kde_pim

Reviewed By: dvratil

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D15975