Eevee: Remove the Volumetric Render checkbox

This is to simplify the usage of Volumetrics.

Now it automatically detect if there is any Volumetric material in the
view and allocate the needed buffer if any.

source/blender/draw/intern/DRW_render.h

@@ -525,6 +525,8 @@ void DRW_pass_sort_shgroup_z(DRWPass *pass);
 
 bool DRW_pass_is_empty(DRWPass *pass);
 
+#define DRW_PASS_CREATE(pass, state) (pass = DRW_pass_create(#pass, state))
+
 /* Viewport */
 typedef enum {
   /* keep in sync with the union struct DRWMatrixState. */