all: remove ineffective _ = variable assignments (#530)

Lastest Zig complains about this, so they must removed to build.

src/platform/native.zig

@@ -290,8 +290,6 @@ pub const Platform = struct {
                     }),
                     else => {},
                 }
-
-                _ = mods;
             }
         }.callback;
         platform.window.setMouseButtonCallback(mouse_button_callback);