Ifdef-out unused shader code in ObjectLightShader.glsl
Remove GL shade model changes

This gets rid of GL state changes for questionable gain. It also fixes drawing
of fade sky backgrounds in global viewports (where, for some reason, the shade
model was set to GL_FLAT instead of GL_SMOOTH).
Render FoW only if active for player

This allows to enable or disable FoW individually for each player with
SetFoW().
Remove objects from light sources when dead or removed
Remove unused FoW tracking in C4Player

The FoW decay mechanism is not needed anymore, either, because the light
system itself is already slowly fading out when the light changes.