Remove unused limitFn in GroupByQuery

[jon-wei]

Removes the unused limitFn in GroupByQuery, as noted here: #3873 (review)

[leventov]

This field is assigned in constructor, and there is non-trivial logic to compute the value for it.

[leventov]

@jon-wei this block and field that you removed in this PR were added just in #3873. Don't we add a bug or regression here? I couldn't answer this question because I don't know what #3873 is about and why this field was added.

[jon-wei]

Looks like that block was reintroduced by #3873 as part of a bad merge that restored some code that was removed by https://github.com/druid-io/druid/pull/4131/files#diff-7564ae1186b5d71c461ade89580058d3.

The limitFn and postProcFn it sets are not being used by anything so the block should be removed entirely.

The equivalent logic is now in GroupByQuery.makePostProcessingFn() and this block in GroupByStrategyV2:

    // Don't apply limit here for inner results, that will be pushed down to the BufferHashGrouper
    if (query.getContextBoolean(CTX_KEY_OUTERMOST, true)) {
      return query.postProcess(rowSequence);
    } else {
      return rowSequence;
    }