Remove legacy --plugin option from flutter create (#12020)

Removed the legacy --plugin option from flutter create.  Fixes #11815.

packages/flutter_tools/lib/src/commands/create.dart

@@ -50,12 +50,6 @@ class CreateCommand extends FlutterCommand {
       },
       defaultsTo: 'app',
     );
-    argParser.addFlag(
-        'plugin',
-        negatable: false,
-        defaultsTo: false,
-        hide: true,
-    );
     argParser.addOption(
       'description',
       defaultsTo: 'A new Flutter project.',
@@ -124,9 +118,7 @@ class CreateCommand extends FlutterCommand {
     if (!fs.isFileSync(fs.path.join(flutterDriverPackagePath, 'pubspec.yaml')))
       throwToolExit('Unable to find package:flutter_driver in $flutterDriverPackagePath', exitCode: 2);
 
-    String template = argResults['template'];
-    if (argResults['plugin'])
-      template = 'plugin';
+    final String template = argResults['template'];
     final bool generatePlugin = template == 'plugin';
     final bool generatePackage = template == 'package';
 

packages/flutter_tools/test/commands/create_test.dart

@@ -116,25 +116,6 @@ void main() {
       );
     }, timeout: const Timeout.factor(2.0));
 
-    testUsingContext('plugin project (legacy)', () async {
-      return _createProject(
-        projectDir,
-        <String>['--no-pub', '--plugin'],
-        <String>[
-          'android/src/main/java/com/yourcompany/flutterproject/FlutterProjectPlugin.java',
-          'ios/Classes/FlutterProjectPlugin.h',
-          'ios/Classes/FlutterProjectPlugin.m',
-          'lib/flutter_project.dart',
-          'example/android/app/src/main/java/com/yourcompany/flutterprojectexample/MainActivity.java',
-          'example/ios/Runner/AppDelegate.h',
-          'example/ios/Runner/AppDelegate.m',
-          'example/ios/Runner/main.m',
-          'example/lib/main.dart',
-        ],
-        plugin: true,
-      );
-    });
-
     testUsingContext('kotlin/swift plugin project', () async {
       return _createProject(
         projectDir,