Remove the variable parent_trx.

Call innobase_register_stmt() in order to work around a MySQL bug
in mysql_alter_table(), which, as of

ChangeSet@1.2482.61.2, 2007-06-07 16:37:15+02:00, joerg@trift2. +8 -0

commits the transaction before calling ha_innobase::add_index().
Without re-registering the statement, the ha_commit_stmt(thd)
in mysql_alter_table() would not invoke innobase_commit.