Remove Support for AreaLight

AreaLights while part of the API are broken in much of the existing rendering paths.  We could fix this, but the techniques used to render real-time AreaLights are very expensive, and don't really make sense for the hardware platforms we see as our targets (Embedded).  So instead we should remove them for 6.0 with the possibility of re-adding support later if there is an actual demand.

Previously in Qt 3D Studio (and Qt Quick 3D by extension) we had support for AreaLights, but not spot lights, and so AreaLights was the only way to have a directional light with falloff.  Since then we have added support for SpotLights which should fulfill this use case without compromising runtime performance.