[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
yes,i know what you mean. preferred locality donot work in our application. i will take a look at it.thanks.