Remove custom styling from button component

Summary:
Remove the custom styling from the FlatButtonWithToolTip component.
The QQC2 desktop style can handle all required features since KF 5.56
Slightly alters the appearance of the media control bar.

Reviewers: mgallien, ngraham

Reviewed By: mgallien, ngraham

Differential Revision: https://phabricator.kde.org/D21634