MR AM to clean up temporary files from previous attempt in case of no recovery

When the MR AM restarts, the {outputDir}/_temporary/{appAttemptNumber} directory remains on HDFS, even though this directory is not used during the next attempt if the restart has been done without recovery. So if recovery is not used for the AM restart, then the deletion of this directory can be done earlier (at the start of the next attempt). The benefit is that more free HDFS space is available for the next attempt.