This patch fixes the bug#21041908 - EVENTS+RPL: VOID CLOSE_THREAD_TABLES(THD*): ASSERTION THD->GET_TRANSACTION()

The issue is related to the automatic dropping of the event (since the end of
event's lifetime) and the use of row based binlogging.

After a row for event being dropped has been deleted from the table
mysql.event the function binlog_log_row is called to record a new entry
into the binlog. In case row based binlogging is on the following
sequence of calls are happen to start a new statement-level transaction:
binlog_log_row --> write_locked_table_maps -->
  THD::binlog_write_table_map --> binlog_start_trans_and_stmt

Latter when open tables are closed the following assert
  DBUG_ASSERT(thd->get_transaction()->is_empty(Transaction_ctx::STMT) ||
              thd->in_sub_stmt ||
              (thd->state_flags & Open_tables_state::BACKUPS_AVAIL));
  is fired since there is a statement-level transaction.

To fix the bug simple refactoring has been done to move turning off
row binlogging inside Event_db_repository::drop_event() that is
called both when DROP EVENT and automatic dropping are executed.