Cycles: Make camera-in-volume an official feature

This means it's no longer needed to enable experimental feature set in order to
have proper camera in volume support. And this also means if there's something
wrong going on, or if there's speed regression for cases when camera is obviously
not in the volume -- this issues are to be reported and handled in the regular
matter.

Happy blending!

intern/cycles/render/camera.h

@@ -102,13 +102,6 @@ public:
 	bool need_device_update;
 	int previous_need_motion;
 
-	/* Camera in volume. */
-	/* TODO(sergey): Get rid of this argument once
-	 * cameras in volume considered fast enough for
-	  * the regular kernel.
-	 */
-	bool use_camera_in_volume;
-
 	/* functions */
 	Camera();
 	~Camera();