Remove Scorer.getChildren? [LUCENE-6229]

This API is used in a single place in our code base: ToParentBlockJoinCollector. In addition, the usage is a bit buggy given that using this API from a collector only works if setScorer is called with an actual Scorer (and not eg. FakeScorer or BooleanScorer like you would get in disjunctions) so it needs a custom IndexSearcher that does not use the BulkScorer API.

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Migrated from [LUCENE-6229](https://issues.apache.org/jira/browse/LUCENE-6229) by Adrien Grand (@jpountz), updated Mar 27 2019
Linked issues:
- #8016
- #8679
- [SOLR-8643](https://issues.apache.org/jira/browse/SOLR-8643)

Start by tracing Scorer.getChildren and its sole reported use in ToParentBlockJoinCollector. Read the linked issues for current context, then inspect how IndexSearcher and BulkScorer affect scorer types and determine the intended removal or replacement. Done means the obsolete API and its usage are resolved without the reported collector and disjunction problems.

Written by the indexing model from the issue text.