Bug#49165 Remove unused/confusing code/comments in setup_conds()

sql/sql_base.cc:
  Query_arena *arena and backup are both unused.
  Removing those, allows us to remove select_lex->conds_processed_with_permanent_arena
sql/sql_lex.cc:
  conds_processed_with_permanent_arena is unused
sql/sql_lex.h:
  conds_processed_with_permanent_arena was unused

sql/sql_base.cc

@@ -8008,7 +8008,6 @@ int setup_conds(THD *thd, TABLE_LIST *tables, TABLE_LIST *leaves,
                 COND **conds)
 {
   SELECT_LEX *select_lex= thd->lex->current_select;
-  Query_arena *arena= thd->stmt_arena, backup;
   TABLE_LIST *table= NULL;	// For HP compilers
   /*
     it_is_update set to TRUE when tables of primary SELECT_LEX (SELECT_LEX
@@ -8024,10 +8023,6 @@ int setup_conds(THD *thd, TABLE_LIST *tables, TABLE_LIST *leaves,
   select_lex->is_item_list_lookup= 0;
   DBUG_ENTER("setup_conds");
 
-  if (select_lex->conds_processed_with_permanent_arena ||
-      arena->is_conventional())
-    arena= 0;                                   // For easier test
-
   thd->mark_used_columns= MARK_COLUMNS_READ;
   DBUG_PRINT("info", ("thd->mark_used_columns: %d", thd->mark_used_columns));
   select_lex->cond_count= 0;
@@ -8096,7 +8091,6 @@ int setup_conds(THD *thd, TABLE_LIST *tables, TABLE_LIST *leaves,
       We do this ON -> WHERE transformation only once per PS/SP statement.
     */
     select_lex->where= *conds;
-    select_lex->conds_processed_with_permanent_arena= 1;
   }
   thd->lex->current_select->is_item_list_lookup= save_is_item_list_lookup;
   DBUG_RETURN(test(thd->is_error()));

sql/sql_lex.cc

@@ -1672,7 +1672,6 @@ void st_select_lex::init_query()
   parent_lex->push_context(&context);
   cond_count= between_count= with_wild= 0;
   max_equal_elems= 0;
-  conds_processed_with_permanent_arena= 0;
   ref_pointer_array= 0;
   select_n_where_fields= 0;
   select_n_having_items= 0;

sql/sql_lex.h

@@ -659,11 +659,6 @@ class st_select_lex: public st_select_lex_node
   uint select_n_where_fields;
   enum_parsing_place parsing_place; /* where we are parsing expression */
   bool with_sum_func;   /* sum function indicator */
-  /* 
-    PS or SP cond natural joins was alredy processed with permanent
-    arena and all additional items which we need alredy stored in it
-  */
-  bool conds_processed_with_permanent_arena;
 
   ulong table_join_options;
   uint in_sum_expr;