chore: update react imports

Signed-off-by: Paul Schultz <pschultz@pobox.com>

docs/tutorials/enable-public-entry.md

@@ -32,7 +32,6 @@ With that, Backstage's cli and backend will detect public entry point and serve
 2. This file is the public entry point for your application, and it should only contain what unauthenticated users should see:
 
    ```tsx title="in packages/app/src/index-public-experimental.tsx"
-   import React from 'react';
    import ReactDOM from 'react-dom/client';
    import { createApp } from '@backstage/app-defaults';
    import { AppRouter } from '@backstage/core-app-api';

docs/tutorials/quickstart-app-plugin.md

@@ -134,7 +134,6 @@ changes, let's start by wiping this component clean.
 1. Replace everything in the file with the following:
 
 ```tsx
-import React from 'react';
 import useAsync from 'react-use/lib/useAsync';
 import Alert from '@material-ui/lab/Alert';
 import { Table, TableColumn, Progress } from '@backstage/core-components';

docs/tutorials/react18-migration.md

@@ -50,7 +50,6 @@ To switch a project to React 18, there are generally three changes that need to
 
 ```tsx title="packages/app/src/index.tsx"
 import '@backstage/cli/asset-types';
-import React from 'react';
 // highlight-remove-next-line
 import ReactDOM from 'react-dom';
 // highlight-add-next-line