Dry up both the two ImpactsEnum Implementation ins Lucene912PostingsR…

…eader (apache#13892)

These two share a lot of code, in particular the impacts implementation is 100% identical.
We can save a lot of code and potentially some cycles for method invocations by
drying things up. The changes are just mechanical field movements with the following exceptions:

1. One of the two implementations was using a bytes ref builder, one a bytes ref for holding the
serialized impacts. The `BytesRef` variant is faster so I used that for both when extracting.
2. Some simple arithmetic simplifications around the levels that should be obvious.
3. Removed the the logic for an index without positions in `BlockImpactsPostingsEnum`, that was dead code,
we only set this thing up if there's positions.