KMessageBox: remove unused translator tuning of button texts

Got broken during preparations of KF in kdelibs, by replacing the "%1" and thus the string to look-up in the catalog directly with code, e.g.

-    dst.setText( i18nc( "@action:button filter-yes", "%1", src.text() ) );
+    dst.setText( i18nc( "@action:button filter-yes", src.text().toUtf8().constData() ) );

possibly missing out the trick used here. See unmaintained/kdelibs@1cedf2c9

As result the string no longer got extracted into the template catalog and thus also removed this entry from translators' faces. Compare the pot of KWidgetsAddons: https://websvn.kde.org/trunk/l10n-kf5/templates/messages/kwidgetsaddons/kwidgetsaddons5_qt.pot?view=markup

The feature was introduced in 2008 by unmaintained/kdelibs@5f9d339f but I found no related discussion or documentation why this was done. Do you remember, @ilic? And what is the purpose of that post-filter hook?

@ltoscano @ilic @ervin