Remove Rdb_transaction::m_is_tx_failed field

[laurynas-biveinis]

Its sole purpose was to handle rollbacks of single-statement autocommit transactions: first, rocksdb_rollback rolled back the statement itself, set the flag, and then ha_rocksdb::external_lock used the flag to roll back the whole transaction.

Simplify this by rolling back the whole transaction in rocksdb_rollback, and letting ha_rocksdb::external_lock to always call commit, which DTRT on rolled-back transactions too.