[BDG-FORMATS-26] Removing bulky fields from the pileup record.

This pull request removes "bulky" fields from the pileup record. The read group fields in the pileup record lead to a large memory/GC overhead and are ultimately unnecessary for most pileup related tasks. This also removes the aggregation specific fields from the pileup record.
If we drop it, we can always add it back later. I think the general wisdom is that aggregators aren't actually that useful (the GATK doesn't recommend using reduceReads anymore). They provide good compression on disk, but pileups aren't a good storage format because they're somewhat lossy. As long as we're careful with what we're storing in memory, there's not that much of a gain (in terms of computational time) with aggregation, IMO.