Remove the background gradient from icon view

Summary:
The icon view showed a very faint gradient in the background. It looked weird, broken and not really like a gradient for some reason. I think it looks better now.
There are some screenshots attached to the bug report.

BUG: 342938
FIXED-IN: 5.15.0

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16998