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

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

superset-frontend/src/explore/components/controls/DatasourceControl/DatasourceControl.test.jsx

@@ -16,7 +16,6 @@
  * specific language governing permissions and limitations
  * under the License.
  */
-import React from 'react';
 import sinon from 'sinon';
 import configureStore from 'redux-mock-store';
 import { mount, shallow } from 'enzyme';

superset-frontend/src/explore/components/controls/DatasourceControl/DatasourceControl.test.tsx

@@ -17,7 +17,6 @@
  * under the License.
  */
 
-import React from 'react';
 import { Route } from 'react-router-dom';
 import fetchMock from 'fetch-mock';
 import userEvent from '@testing-library/user-event';

superset-frontend/src/explore/components/controls/DatasourceControl/index.jsx

@@ -18,7 +18,7 @@
  * under the License.
  */
 
-import React from 'react';
+import { PureComponent } from 'react';
 import PropTypes from 'prop-types';
 import {
   DatasourceType,
@@ -171,7 +171,7 @@ const preventRouterLinkWhileMetaClicked = evt => {
   }
 };
 
-class DatasourceControl extends React.PureComponent {
+class DatasourceControl extends PureComponent {
   constructor(props) {
     super(props);
     this.state = {