sql/sql_class.cc:
  Statement constructor for the case when it's used for backup only
  was removed.
sql/sql_class.h:
  Remove THD::stmt_backup and simplify Statement constructors.
sql/sql_prepare.cc:
  Use an object on stack instead of THD::stmt_backup
sql/sql_select.cc:
  Use an object on stack instead of THD::stmt_backup