fix: result cleanCookies in iOS (#585)

ios/Classes/FlutterWebviewPlugin.m

@@ -62,7 +62,9 @@ static NSString *const CHANNEL_NAME = @"flutter_webview_plugin";
         [self stopLoading];
         result(nil);
     } else if ([@"cleanCookies" isEqualToString:call.method]) {
-        [self cleanCookies];
+        [[NSURLSession sharedSession] resetWithCompletionHandler:^{
+            result(nil);
+        }];
     } else if ([@"back" isEqualToString:call.method]) {
         [self back];
         result(nil);
@@ -289,11 +291,6 @@ static NSString *const CHANNEL_NAME = @"flutter_webview_plugin";
     }
 }
 
-- (void)cleanCookies {
-    [[NSURLSession sharedSession] resetWithCompletionHandler:^{
-        }];
-}
-
 - (bool)checkInvalidUrl:(NSURL*)url {
   NSString* urlString = url != nil ? [url absoluteString] : nil;
   if (_invalidUrlRegex != [NSNull null] && urlString != nil) {