upgrade to Filament 1.21.0

ios/Classes/PolyvoxFilamentPlugin.m

@@ -1,15 +0,0 @@
-#import "PolyvoxFilamentPlugin.h"
-#if __has_include(<polyvox_filament/polyvox_filament-Swift.h>)
-#import <polyvox_filament/polyvox_filament-Swift.h>
-#else
-// Support project import fallback if the generated compatibility header
-// is not copied when this plugin is created as a library.
-// https://forums.swift.org/t/swift-static-libraries-dont-copy-generated-objective-c-header/19816
-#import "polyvox_filament-Swift.h"
-#endif
-
-@implementation PolyvoxFilamentPlugin
-+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
-  [SwiftPolyvoxFilamentPlugin registerWithRegistrar:registrar];
-}
-@end