Revert "Maintain sort order when new articles are added"

While fixing the new article sorting issue, this introduced regressions
when displaying only unread articles.

Alex, please fix and resubmit to kdepim/4.6.
BUG: 271794
BUG: 271149
CCBUG: 271640
FIXED-IN: kdepim 4.6

This reverts commit e95e0f8e.