GlobalToolBar: Remove superfluous piece of background
It used to be stretched vertically, but that started conflicting with vertical centering anchors, so in attempts to fix that it got broken and nothing is there to give it any height anymore.
Other than that, Kirigami.Theme.backgroundColor should be a default implementation of AbstractApplicationHeader::background on any theme, so there is no need to repeat it here, especially with such custom size.
Besides, this binding on width is 100% broken on RTL anyway.