[CT-2217] [Redshift] Remove constraints_check warning in get_columns_spec_ddl

[MichelleArk]

Core issue: dbt-labs/dbt-core#7067

dbt-redshift can use the default implementation of `constraint_support to model its enforced/support status of constraints.

Once the warning mechanism has been generalized by dbt-labs/dbt-core#7067, we can remove the warning raised in get_columns_spec_ddl: https://github.com/dbt-labs/dbt-redshift/blob/main/dbt/include/redshift/macros/utils/get_columns_spec_ddl.sql#L22-L25