This tracks a couple of cleanup issues that were identified in
- It seems unnecessary to pass the job token and credentials separately when we always combine the job token into the credentials before building the container launch context. The TaskImpl and TaskAttemptImpl constructors could simply take credentials with the job token already added rather than separate job token and credential parameters.
- It's unclear whether we still need the appTokens file that is placed into HDFS by the job submitter, localized by the NM, and finally read in by the AM. I believe the AM's credentials sent in the AM's container launch context already contains the same information. If that's the case, we should remove the code related to the appTokens file.