Remove TODO file

Summary:
Only these items from the TODO file have some relevance

- Open games in tabs - tabs (will) have tear-off functionality - reported here: https://bugs.kde.org/show_bug.cgi?id=394596
- Add logo in background when exporting / printing - reported here: https://bugs.kde.org/show_bug.cgi?id=394597
- Extend export print functionality (multiple pages) - see also https://bugs.kde.org/show_bug.cgi?id=168622
- Add world-wide high scores - already reported here: https://bugs.kde.org/show_bug.cgi?id=168615
- Add a puzzle-shape editor downloader & uploader - reported here: https://bugs.kde.org/show_bug.cgi?id=394598

I checked with the bug list (https://bugs.kde.org/buglist.cgi?bug_status=UNCONFIRMED&bug_status=CONFIRMED&bug_status=ASSIGNED&bug_status=REOPENED&bug_status=NEEDSINFO&bug_status=VERIFIED&columnlist=product%2Ccomponent%2Cassigned_to%2Cbug_severity%2Cbug_status%2Cresolution%2Cshort_desc%2Cchangeddate&list_id=1517401&product=ksudoku&query_format=advanced) if those are already present and added reports for missing items.

Reviewers: #kde_games

Subscribers: aacid, kde-games-devel

Differential Revision: https://phabricator.kde.org/D12922