Mantaflow: Address precompiler warning and related cleanup

* Address warning because of undefined OPENVDB usage
* Remove unused WITH_FLUID definitions

Differential Revision: https://developer.blender.org/D6919

source/blender/render/CMakeLists.txt

@@ -86,10 +86,6 @@ if(WITH_IMAGE_OPENEXR)
   add_definitions(-DWITH_OPENEXR)
 endif()
 
-if(WITH_MOD_FLUID)
-  add_definitions(-DWITH_FLUID)
-endif()
-
 if(WITH_FREESTYLE)
   list(APPEND INC
     ../freestyle