remove "parent" from classification

"parent" is a leftover from a feature that was planned 5 years ago but
was never implemented. To have something like a "tree view" for
savegames in the load dialog.

"parent" gets written but never gets read.

The current code writing the "parent" doesn't really work because it is
only takes the name of the last savefile unrelated of whether there was
a reload, or similar between so for example a "parent" of an sp campaign
save could easily be a mp match save. Also it could produce cycle which
would also be wrong for a tree view.