RFC: KDebugSettings data: drop KF5-time old category names

Writing this text made me rethink things, and I currently think this removal is wrong. But let's have someone else comment on this who actually knows, like @mlaurent :)

As both Qt5 and Qt6 share the config files and environment variables, unless I misread things, KDebugSettings would still prefer to know any old names possibly used in .config/QtProject/qtlogging.ini to match them to entries in share/qlogging-categories6/*.categories, right? So if someone switches from some old KF5-based version to latest KF6 one, KDebugSettings would be able to still map any existing config settings to latest category names.

So this patch would be bad as it removes useful information, right?