Remove unused variables (#3906)

app/javascript/mastodon/features/compose/index.js

@@ -1,6 +1,5 @@
 import React from 'react';
 import ComposeFormContainer from './containers/compose_form_container';
-import UploadFormContainer from './containers/upload_form_container';
 import NavigationContainer from './containers/navigation_container';
 import PropTypes from 'prop-types';
 import { connect } from 'react-redux';