Remove KCoreDirListerCache::validUrl, let the job emit error instead

Because of these early checks (before creating a list job), KCoreDirLister has a special handleErrorMessage(QString) in addition to handleError(KJob*). Skip those checks, the job will error properly in those two unlikely cases (invalid URL, non listable URL).

For the next step, this will allow to have a single signal (instead of the handleError virtual) and emit a valid KJob* in that signal.

While at it, add KF6 TODOs about removing 'virtual' for most methods in KCoreDirLister (this comes from very old days of making everything virtual just in case, but it makes no sense, none of these have access to any of the data, in a subclass).

KF6 task: https://phabricator.kde.org/T11821