Remove HashDocSet

This particular DocSet is only used in places where we need to convert SortedIntDocSet in particular to a DocSet that is fast for random access. Once such a conversion happens, it's only used to test some docs for presence and it could be another interface. DocSet has kind of a large-ish API surface area to implement. Since we only need to test docs, we could use Bits interface (having only 2 methods) backed by an off-the-shelf primitive long hash set on our classpath. Perhaps a new method on DocSet: getBits() or DocSetUtil.getBits(DocSet).

In addition to removing complexity unto itself, this improvement is required by SOLR-14185 because it wants to be able to produce a DocIdSetIterator slice directly from the DocSet but HashDocSet can't do that without sorting first.