upgrade to Filament 1.21.0

ios/Classes/SwiftPolyvoxFilamentPlugin.swift

@@ -1,14 +0,0 @@
-import Flutter
-import UIKit
-
-public class SwiftPolyvoxFilamentPlugin: NSObject, FlutterPlugin {
-  public static func register(with registrar: FlutterPluginRegistrar) {
-    let channel = FlutterMethodChannel(name: "app.polyvox.filament", binaryMessenger: registrar.messenger())
-    let instance = SwiftPolyvoxFilamentPlugin()
-    registrar.addMethodCallDelegate(instance, channel: channel)
-  }
-
-  public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
-    result("iOS " + UIDevice.current.systemVersion)
-  }
-}