Drop unused broken KGesture support

Summary:
The functionality was disabled before kdelibs 3.5!
(7a390751076bafafc67557007172028956966ef1)

These classes were compile time skipped from KShortcutsEditor and
KShortcutsEditorItem and unused elsewhere in the library.

Gestures exist in some form in Qt.

The header files were installed as _p.h

The symbols were exported, but anyone using them was using clearly
private API.

Test Plan: Compiles

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: apol, vkrause, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25511