How can I remove the Flutter debug banner?

On your MaterialApp set debugShowCheckedModeBanner to false.

MaterialApp(
  debugShowCheckedModeBanner: false,
)

Or, on your CupertinoApp:

CupertinoApp(
  debugShowCheckedModeBanner: false
)

The debug banner will also automatically be removed on release build. If you are using Android Studio then go to Run → Flutter → Run ‘main.dart’ in Release mode.