applet: Drop redundant workaround for semi-transparent sliders
It may have served its purpose with dignity, but is no longer needed in
the presence of layer.enabled: opacity < 1 binding at the control level.
See also: https://doc.qt.io/qt-5/qml-qtquick-item.html#item-layers
Note: this MR is based on !135 (merged), so there are actually two commits until that one is merged first.
Edited by ivan tkachenko