Remove QPlatformScreen::screenGeometry()

QScreen's geometry is always determined by the platform screen,
since there is no Qt API to resize or move a screen. Therefore,
it makes no sense for the platform screen to ask QScreen for the
geometry.

Change-Id: Iedaa40e06e3a878f4e4e13fddd13a67a62d1f0f9
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>