When Krusader users utilize the "Search for" comboBox on the "Locate" dialog:
they often see bad entries because there are obsolete ones, 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.

With a KrHistoryComboBox, Krusader users are able to delete wrong entries from
history lists, and therefore they can remove those bad suggestions that they
were seeing.

Open Krusader, go to "Tools > Locate...", check that the "Locate" dialog
appears. Type a word. Press "Locate". Close Krusader. Check that in the
~/.config/krusaderrc file: after Search For= is seen the previously typed
word.

Open Krusader again. Go to the "Locate" dialog, delete the previously typed word
from the comboBox:
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.

Try to delete all the typed commands until the list is empty, press Shift+Del
again. Close Krusader. Monitor the changes in the ~/.config/krusaderrc file.

Perform similar experiments with other words, having more words available in the
comboBox.

Note: Those changes are designed to be tried after applying the ones
described in https://phabricator.kde.org/D28999