Remove obsolte and broken importer

Summary:

• tuxkarts is dead since 2010
• KnowIt is from KDE3
• knotes an kjots importer were written before Akonadi so doesn't work on the current version of these softwares
• Sticky Notes was last updated 14 years ago
• tomboy doesn't exist anymore but there is tomboy-ng. maybe the file format is still compatible
• And I didn't test Jreepad but I don't think there are many Jreepad users

Test Plan: Compile and see the menu with less entries

Reviewers: niccolove

Reviewed By: niccolove

Subscribers: anthonyfieroni

Differential Revision: https://phabricator.kde.org/D29559