Cleanup delete handling issues in POP3

* Removed obsolete "delete after 7 days" option from pop-up prefs
* Mark deleted message sentinels as "read" so they don't contribute
    to unread counts.

Bug # 2157487 and Bug # 2159278

Change-Id: Ic3cbc51d6f5ede2eb923e2d0e5c0dfee377764f5
1 file changed