Use the new JSX transform (#25064)

app/javascript/mastodon/features/ui/components/column_header.jsx

@@ -1,9 +1,9 @@
-import React from 'react';
+import { PureComponent } from 'react';
 import PropTypes from 'prop-types';
 import classNames from 'classnames';
 import { Icon }  from 'mastodon/components/icon';
 
-export default class ColumnHeader extends React.PureComponent {
+export default class ColumnHeader extends PureComponent {
 
   static propTypes = {
     icon: PropTypes.string,