chore: remove React 16.4's obsolete React imports (#28571)

Signed-off-by: hainenber <dotronghai96@gmail.com>

superset-frontend/src/explore/components/ExploreChartHeader/ExploreChartHeader.test.tsx

@@ -17,7 +17,6 @@
  * under the License.
  */
 
-import React from 'react';
 import sinon from 'sinon';
 import { render, screen, waitFor } from 'spec/helpers/testing-library';
 import userEvent from '@testing-library/user-event';

superset-frontend/src/explore/components/ExploreChartHeader/index.jsx

@@ -16,7 +16,7 @@
  * specific language governing permissions and limitations
  * under the License.
  */
-import React, { useCallback, useEffect, useMemo, useState } from 'react';
+import { useCallback, useEffect, useMemo, useState } from 'react';
 import { useHistory } from 'react-router-dom';
 import { useDispatch } from 'react-redux';
 import PropTypes from 'prop-types';