MapTask doesn't need to put username information in SpillRecord

This is an amendment to MAPREDUCE-2096 that's found in Yahoo's 0.20.100 branch.

This bug causes task failures in the following case:

• Cluster is not set up with LinuxTaskController (ie not secured cluster)
• Job submitter is not the same as the user running the TT
• Map output is more than one spill's worth

The issue is that UserGroupInformation's view of the current user is the job submitter, but on disk the spill files will be owned by the TT user. SecureIO will then fail when constructing the spill record.