chore: update react imports

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

plugins/signals/dev/index.tsx

@@ -13,7 +13,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import React from 'react';
 import { createDevApp } from '@backstage/dev-utils';
 import { signalsPlugin } from '../src/plugin';
 import { CodeSnippet, Content, Header, Page } from '@backstage/core-components';