presto-main/src/main/java/io/prestosql/execution/QueryStateMachine.java/QueryStateMachine/removePreparedStatement
class QueryStateMachine:    public void removePreparedStatement(String key)
    {
        requireNonNull(key, "key is null");

        if (!session.getPreparedStatements().containsKey(key)) {
            throw new PrestoException(NOT_FOUND, "Prepared statement not found: " + key);
        }
        deallocatedPreparedStatements.add(key);
    }