batch: remove re-export of consistency enums

Remove re-export of consistency enums in batch.rs:
- scylla:statement::batch::Consistency
- scylla:statement::batch::SerialConsistency

A user of the driver should instead use:
- scylla:statement::Consistency
- scylla:statement::SerialConsistency

This change is a part of our effort to stabilize the API and reduce
the number of pub exports.

Refs #660

scylla/src/statement/batch.rs

@@ -6,7 +6,7 @@ use crate::statement::{prepared_statement::PreparedStatement, query::Query};
 use crate::transport::execution_profile::ExecutionProfileHandle;
 
 use super::StatementConfig;
-pub use super::{Consistency, SerialConsistency};
+use super::{Consistency, SerialConsistency};
 pub use crate::frame::request::batch::BatchType;
 
 /// CQL batch statement.