Omit computing max score when sorting by score primarily in TopGroupsCollector

[gaobinlong]

Description

In TopGroupsCollector, omit MaxScoreCollector when withinGroupSort has score as the primary field. The group max score is instead derived from FieldDoc.fields[0] (set by RelevanceComparator), avoiding the overhead of a parallel score-tracking collector.

This helps improve the performance of grouping search, benchmark on wikimedium1m:

                        TaskQPS baseline      StdDevQPS my_modified_version      StdDev                Pct diff p-value
                TermGroup10K      124.24     (12.3%)      126.94     (11.5%)    2.2% ( -19% -   29%) 0.480
                TermGroup100      117.89     (31.3%)      120.88     (26.7%)    2.5% ( -42% -   88%) 0.736
                 TermGroup1M      118.59     (10.9%)      122.83     (13.8%)    3.6% ( -19% -   31%) 0.265