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/nodes/function/node_function_util.cc

@@ -5,7 +5,7 @@
 
 #include "NOD_socket_search_link.hh"
 
-static bool fn_node_poll_default(bNodeType *UNUSED(ntype),
+static bool fn_node_poll_default(bNodeType * /*ntype*/,
                                  bNodeTree *ntree,
                                  const char **r_disabled_hint)
 {