GRAINS-U.S. wheat drops on profit-taking, corn also weak

* Wheat falls after hitting highest since June 30 * Low export demand weighs on corn * Soy mixed, with new-crop pressured by crop expectations (Recasts with updated U.S. prices and additional quote; changes byline and dateline) By Mark Weinraub CHICAGO, Aug 7 (Reuters) – U.S. wheat futures fell on Thursday, dropping back from a