MDEV-29988: (revert) Major performance regression with 10.6.11

Reverted changed in server code introduced by
the commit bd9274fa.
Tests from this commit are retained.

sql/item_cmpfunc.cc

@@ -442,18 +442,9 @@ bool Item_func::setup_args_and_comparator(THD *thd, Arg_comparator *cmp)
   if (args[0]->cmp_type() == STRING_RESULT &&
       args[1]->cmp_type() == STRING_RESULT)
   {
-    Query_arena *arena, backup;
-    arena= thd->activate_stmt_arena_if_needed(&backup);
-
     DTCollation tmp;
-    bool ret= agg_arg_charsets_for_comparison(tmp, args, 2);
-
-    if (arena)
-      thd->restore_active_arena(arena, &backup);
-
-    if (ret)
-      return ret;
-
+    if (agg_arg_charsets_for_comparison(tmp, args, 2))
+      return true;
     cmp->m_compare_collation= tmp.collation;
   }
   //  Convert constants when compared to int/year field