Remove iOS variant

ios/Runner/main.m

-#import <Flutter/Flutter.h>
-#import <UIKit/UIKit.h>
-#import "AppDelegate.h"
-
-int main(int argc, char* argv[]) {
-  @autoreleasepool {
-    return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
-  }
-}