[SPARK-2687] [yarn]amClient should remove ContainerRequest

in https://issues.apache.org/jira/browse/YARN-1902, after receving allocated containers,if amClient donot remove ContainerRequest,RM will continually allocate container for spark AppMaster.@tgravescs
i donot think so. because it is FIFO, we can remove head of request queue because head of queue is allocated by Yarn when we receive allocated Containers.