Remove crosshair cursor from crop tool

Summary:
After e97eda1df871 made custom cursors set by tools visible again,
seldomly for the Crop tool a crosshair cursor was shown. This
special cursor was added in dbf40f37655e, but due to the inital size of
the selection to cover the complete image by default, there will only
ever be the need for a cursor indicating resizing or moving.

Until Gwenview gains the capability of creating a completely new
selection by dragging with the mouse, let's remove the special cursor
for now.

Test Plan:
Move cursor over image, activate Crop by using the ⇧+C keyboard
shortcut, notice cursor does not turn into crosshair anymore.

Reviewers: Gwenview, muhlenpfordt

Reviewed By: Gwenview, muhlenpfordt

Subscribers: muhlenpfordt

Differential Revision: https://phabricator.kde.org/D14406