Always remove the workspace settings (#56703)

packages/flutter_tools/lib/src/ios/migrations/xcode_build_system_migration.dart

@@ -28,14 +28,10 @@ class XcodeBuildSystemMigration extends IOSMigrator {
 
     final String contents = _xcodeWorkspaceSharedSettings.readAsStringSync();
 
-    // Only delete this file when it matches the original Flutter template.
+    // Only delete this file when it is pointing to the legacy build system.
     const String legacyBuildSettingsWorkspace = '''
-<plist version="1.0">
-<dict>
 	<key>BuildSystemType</key>
-	<string>Original</string>
-</dict>
-</plist>''';
+	<string>Original</string>''';
 
     // contains instead of equals to ignore newline file ending variance.
     if (contents.contains(legacyBuildSettingsWorkspace)) {

packages/flutter_tools/test/general.shard/ios/ios_project_migration_test.dart

@@ -322,6 +322,8 @@ keep this 2
 <dict>
 	<key>BuildSystemType</key>
 	<string>Original</string>
+	<key>PreviewsEnabled</key>
+	<false/>
 </dict>
 </plist>''';
         xcodeWorkspaceSharedSettings.writeAsStringSync(contents);