Remove JOIN::need_distinct

The member was assigned in JOIN::init(), JOIN::optimize_inner() and
JOIN::optimize_stage2(), but never read. Its last reads, the arguments
of prepare_sum_aggregators() calls, were removed by:
commit 2cfc450bf7
Author: Igor Babaev <igor@askmonty.org>
Date:   Tue Feb 9 2016

    This is the consolidated patch for mdev-8646: "Re-factor the code
    for post-join operations".

Since then the callers compute the value themselves from
is_using_agg_loose_index_scan().

JOIN::prepare_sum_aggregators() has a parameter of the same name, which
is unrelated and is kept.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

sql/sql_select.cc

@@ -530,7 +530,6 @@ void JOIN::init(THD *thd_arg, List<Item> &fields_arg,
   simple_order= 0;
   simple_group= 0;
   ordered_index_usage= ordered_index_void;
-  need_distinct= 0;
   skip_sort_order= 0;
   with_two_phase_optimization= 0;
   save_qep= 0;
@@ -2247,7 +2246,6 @@ JOIN::optimize_inner()
   do_send_rows = (unit->lim.get_select_limit()) ? 1 : 0;
 
   set_allowed_join_cache_types();
-  need_distinct= TRUE;
 
   Json_writer_object trace_wrapper(thd);
   Json_writer_object trace_prepare(thd, "join_optimization");
@@ -3589,7 +3587,6 @@ int JOIN::optimize_stage2()
     tmp_table_param.precomputed_group_by= TRUE;
     if (join_tab->is_using_agg_loose_index_scan())
     {
-      need_distinct= FALSE;
       tmp_table_param.precomputed_group_by= FALSE;
     }
   }

sql/sql_select.h

@@ -1609,7 +1609,6 @@ public:
      group_list)
   */
   bool	   group;
-  bool     need_distinct;
 
   /**
     Indicates that grouping will be performed on the result set during