chore: remove unnecessary variable assignments

Author: khatruong2009

packages/auth/amplify_auth_cognito/example/integration_test/mfa_email_optional_test.dart

@@ -18,7 +18,7 @@ void main() {
         final username = env.generateUsername();
         final password = generatePassword();
 
-        final user = await adminCreateUser(
+        await adminCreateUser(
           username,
           password,
           autoConfirm: true,

packages/auth/amplify_auth_cognito/example/integration_test/mfa_username_login_required_test.dart

@@ -230,7 +230,7 @@ void main() {
               .equals(AuthSignInStep.continueSignInWithMfaSelection);
 
           // select totp as the preferred method
-          final selectRes2 = await Amplify.Auth.confirmSignIn(
+          await Amplify.Auth.confirmSignIn(
             confirmationValue: 'TOTP',
           );