Revert "Revert "Do not draw background or line in toolbars if the color scheme...

Revert "Revert "Do not draw background or line in toolbars if the color scheme has no headers group""

This reverts commit 9f40b17e.

The idea of a Tools Area separator only makes sense when there is a unibody
Tools Area. When using a color scheme without Header colors, there is no
Tools Area (just a disparate collection of titlebars, manubars, and
toolbars), so the line is just extra visual noise that various people
have objected to following the Plasma 5.21 release. Let's make it
conditional on using a color scheme with Header colors again.

BUG: 433118
FIXED-IN: 5.21.1