When Krusader users utilize some comboBoxes on the Synchronizer dialog: they
often see bad suggestions because there are obsolete entries, mistakes and so
on, but normal Krusader users can't remove those entries. For example, web
browsers have Shift+Del in order to remove the thwarting entries.

If the KrHistoryComboBox class is used, then Krusader users are able to delete
wrong entries from history lists, and therefore they can remove those bad
suggestions that they were seeing.

Press Ctrl+Y to see the Synchronizer dialog. Using the "Target folder" comboBox
on the Synchronizer dialog, several times: add a new folder entry, synchronize
using it, delete the folder entry:
a) using Shift+Del when the popup list is seen;
b) or using the arrow keys and Shift+Del when the popup list is not visible;
monitor the changes in the ~/.config/krusaderrc file. Try to delete all the
folder entries until the list is empty, press Shift+Del again.

Repeat the same steps for the "Source folder" and for the "File filter"
comboboxes.

Synchronize a folder, just in case.

Note: Those changes are designed to be tried after applying the ones
described in https://phabricator.kde.org/D28962