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/modifiers/intern/MOD_particlesystem.cc

@@ -249,7 +249,7 @@ static void deformVertsEM(ModifierData *md,
 }
 #endif
 
-static void panel_draw(const bContext *UNUSED(C), Panel *panel)
+static void panel_draw(const bContext * /*C*/, Panel *panel)
 {
   uiLayout *layout = panel->layout;