Chris Brown lost his marbles on “GMA,” and now the hot-headed singer has lost his publicist. We hear that Brown’s spokeswoman, Tammy Brook, will no longer be representing him after the star trashed his GMA dressing room, smashed a window and left without his shirt from the morning talk show earlier this week. Sources close to the star tell us she and Brown have decided to end their relationship following his meltdown on “GMA.” Brook couldn’t be reached last night.