Allow resetting a Rectangle gradient.

If a gradient and a color are set on a rectangle, the gradient is used.
This means that if you wish to override the gradient on a component with
a color, you have to unset the gradient.

Also remove the unused QQuickGradient::gradient() method.

Task-number: QTBUG-23238
Change-Id: Ibd43cfe1bd4b867e4f6103f1d0dc0ed6176ab5c1
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>