Remove unnecessary $FlowFixMe

Summary: No longer necessary, after flow types for `lodash.memoize` were added.

Reviewed By: jknoxville

Differential Revision: D15738356

fbshipit-source-id: 3c911e5eefee347ca69a37051fd5279c5de47873

headless-tests/__tests__/headlessIntegrationTests.js

@@ -6,7 +6,6 @@
  */
 
 import {spawn} from 'child_process';
-// $FlowFixMe
 import memoize from 'lodash.memoize';
 
 const TEST_TIMEOUT_MS = 30 * 1000;