If you want to remove the shadow of all app bars without repeating code, just add a AppBarTheme property with elevation: 0 to your app theme (ThemeData), inside your MaterialApp widget:
// This code should be located inside your "MyApp" class, or equivalent (in main.dart by default)
return MaterialApp(
// App Theme:
theme: ThemeData(
// ••• ADD THIS: App Bar Theme: •••
appBarTheme: AppBarTheme(
elevation: 0, // This removes the shadow from all App Bars.
)
),
);