Core: Cleanup non nested filter to not flip the FixedBitSet returned by the wrapped filter.

If the filter producing the FBS were to be cached then it would flip bits each time the NonNestedDocsFilter was executed.

In our case we cached the NonNestedDocsFilter itself so flipping bits each time NonNestedDocsFilter was used  didn't happen. However the hashcode of NonNestedDocsFilter and NestedDocsFilter was the same, since NonNestedDocsFilter directly used NestedDocsFilter#hashCode()

PR for #8227