WASHINGTON—U.S. Rep. Mac Thornberry, R-Clarendon, announced Monday that he will not seek reelection in 2020, making him the sixth GOP congressman from Texas to say he’s retiring in recent weeks.  “It has been a great honor for me to represent the people of the 13th District of Texas for the last 25 years,” he said in a statement.   “We are reminded, however, that ‘for everything, there is a…