Remove empty zorder
Thanks Louis Taylor for the patch!
Summary:
This was causing:
/home/kragniz/git/krita/src/libs/ui/forms/wdgpaintopsettings.ui: Warning: Z-order assignment: '' is not a valid widget.
Reviewers: Krita, dkazakov
Reviewed By: Krita, dkazakov
Subscribers: dkazakov
Differential Revision: https://phabricator.kde.org/D5896