By Kylie Madry MEXICO CITY (Reuters) -Mexican Economy Minister Tatiana Clouthier stepped down on Thursday, depriving Mexico of one of its key trade negotiators as her government scrambles to head off a major dispute over energy with the United States and Canada. Clouthier, a scion of a powerful political family who became economy minister early last year, said she had first told President Andres Manuel Lopez Obrador she intended to quit not long after the energy disagreement threatened to become costly for Mexico. Her resignation was effective immediately. The United States requested dispute s…