Fix ascending/descending choosers getting unchecked when re-selecting the same sort order

Summary:
When you select the same sort order that's already selected, the currently-checked
sort order description (the human-readable ascending/descending items) gets unchecked
in `slotSortTriggered()` yet the ascending/descending items items only get checked in
`slotSortOrderChanged()`. Because the order hasn't gotten changed, neither one gets
checked again.

This patch fixes the problem by not unchecking them in the first place.

BUG: 411223
FIXED-IN: 19.08.2

Test Plan:
1. Right-click > Sort By > Click the currently-selected sort order
2. Right-click > Sort By > See that the item for the current ascending/descending setting has not been changed

Reviewers: elvisangelaccio, #dolphin

Reviewed By: elvisangelaccio, #dolphin

Subscribers: broulik, meven, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D23412