Cleanup: replace UNUSED macro with commented args in C++ code

This is the conventional way of dealing with unused arguments in C++,
since it works on all compilers.

Regex find and replace: `UNUSED\((\w+)\)` -> `/*$1*/`

source/blender/blenlib/intern/mesh_boolean.cc

@@ -2527,7 +2527,7 @@ class InsideShapeTestData {
 static void inside_shape_callback(void *userdata,
                                   int index,
                                   const BVHTreeRay *ray,
-                                  BVHTreeRayHit *UNUSED(hit))
+                                  BVHTreeRayHit * /*hit*/)
 {
   const int dbg_level = 0;
   if (dbg_level > 0) {