Cleanup KSudoku::updateShapesList() to remove manual path fiddling.

Summary: This change resolves Phabricator/Maniphest task T5977.

Test Plan:
Compiled & tested I could still generate custom game variant puzzles: Mathdoku 4x4, Killer 9x9.

Also, to confirm the duplicate filtering is redundant I ran ksudoku with a deliberately messed up XDG_DATA_DIRS environment variable:

XDG_DATA_DIRS="$XDG_DATA_DIRS:$PWD/kde-5/share;$PWD/kde-5/share;$PWD/kde-5/share" ./kde-5/bin/ksudoku

(Which still lists each game variant type only once, meaning the filtering was in fact redundant.)

Reviewers: KDE Games, ltoscano

Reviewed By: ltoscano

Subscribers: ltoscano, KDE Games

Maniphest Tasks: T5977

Differential Revision: https://phabricator.kde.org/D5669