remove unused loader callback on macOS

macos/Classes/SwiftPolyvoxFilamentPlugin.swift

@@ -148,8 +148,6 @@ public class SwiftPolyvoxFilamentPlugin: NSObject, FlutterPlugin, FlutterTexture
             let metalTexturePtr = Unmanaged.passUnretained(metalTexture!).toOpaque()
             let metalTextureAddress = Int(bitPattern:metalTexturePtr)
                                                                                             
-            let callback = make_resource_loader(loadResource, freeResource,  Unmanaged.passUnretained(self).toOpaque())
-                                                                                            
             result([self.flutterTextureId as Any, nil, metalTextureAddress])
         case "destroyTexture":
             if(self.flutterTextureId != nil) {