Remove Q_UNREACHABLE from DesktopScreenQt::GetDisplayNearestWindow

Chromium uses this method when the user attempts to drag.

Task-number: QTBUG-45723
Change-Id: Ibe3946846f0f4e2d2a25ca36e66fa7235d4aecea
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>