presto-main/src/main/java/io/prestosql/execution/DeallocateTask.java/DeallocateTask/execute
class DeallocateTask:    @Override
    public ListenableFuture<?> execute(Deallocate statement, TransactionManager transactionManager, Metadata metadata, AccessControl accessControl, QueryStateMachine stateMachine, List<Expression> parameters)
    {
        String statementName = statement.getName().getValue();
        stateMachine.removePreparedStatement(statementName);
        return immediateFuture(null);
    }