unnecessary assert on docid [LUCENE-4958]

In DocFieldProcessor, on line 353, there is this assert:

assert docValuesConsumerAndDocID.docID < docState.docID;

Is this assert necessary? I don't see in the indexing pipeline that this guarantee is needed. Can we remove this?

We have implemented a custom indexingchain that rewrites docState.docID is reverse order and it is working well. But we have to do ugly workarounds in our test to avoid this assert.

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Migrated from [LUCENE-4958](https://issues.apache.org/jira/browse/LUCENE-4958) by John Wang

Inspect DocFieldProcessor around line 353 and trace the indexing pipeline to determine whether the docID ordering assertion is required. Remove the assertion if reverse-order docState.docID values remain valid, then run the relevant indexing tests and confirm the existing workaround is no longer needed.

Written by the indexing model from the issue text.