remove handler::open_read_view()

ht->start_consistent_snapshot() is also not a way,
because some engines (e.g. rocksdb) only do it readonly.
instead, downgrade the lock after reading the first row
(which implicitly opens a read view).

sql/handler.h

@@ -3562,7 +3562,6 @@ class handler :public Sql_alloc
   /** to be actually called to get 'check()' functionality*/
   int ha_check(THD *thd, HA_CHECK_OPT *check_opt);
   int ha_repair(THD* thd, HA_CHECK_OPT* check_opt);
-  virtual void open_read_view(){}
   void ha_start_bulk_insert(ha_rows rows, uint flags= 0)
   {
     DBUG_ENTER("handler::ha_start_bulk_insert");

storage/innobase/handler/ha_innodb.cc

@@ -21165,17 +21165,3 @@ buf_pool_size_align(
     return (size / m + 1) * m;
   }
 }
-
-void ha_innobase::open_read_view()
-{
-  trx_t *trx= m_prebuilt->trx;
-  auto thd_iso= thd_get_trx_isolation(m_user_thd);
-
-  trx->isolation_level= innobase_map_isolation_level(thd_iso);
-  ut_ad(trx->isolation_level == TRX_ISO_REPEATABLE_READ);
-  ut_ad(!trx_is_started(trx));
-
-  trx_start_if_not_started_xa(trx, false);
-
-  trx->read_view.open(m_prebuilt->trx);
-}

storage/innobase/handler/ha_innodb.h

@@ -438,7 +438,6 @@ class ha_innobase final : public handler
 			  const KEY_PART_INFO& old_part,
 			  const KEY_PART_INFO& new_part) const override;
 
-        void open_read_view() override;
 protected:
 	bool
 	can_convert_string(const Field_string* field,