_PyMem_DebugFree(): fix compiler warning on Windows

Don't return a void value.
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index 503fcdf..40c9fcd 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -2039,7 +2039,7 @@
 _PyMem_DebugFree(void *ctx, void *ptr)
 {
     _PyMem_DebugCheckGIL();
-    return _PyMem_DebugRawFree(ctx, ptr);
+    _PyMem_DebugRawFree(ctx, ptr);
 }
 
 static void *