koprefsdialog.cpp - less clutter in holidays combobox

Summary:
don't use the description for the holidays. restore
the old way of using only the regionName and language.

if any of the holiday region list entries are missing
subdivisions or otherwise incorrect it probably means
there is some info missing from there holiday file.

Reviewers: vkrause, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29576