docs: remove legacy config option, close #703

Author: antfu

Diff for: docs/guide/config-presets.md

@@ -20,7 +20,6 @@ import stylistic from '@stylistic/eslint-plugin'
 
 export default [
   stylistic.configs.customize({
-    flat: true, // required for flat config
     // the following options are the default values
     indent: 2,
     quotes: 'single',