[Feature] Remove `dbt-postgres` dependency about dbt-redshift

[colin-rogers-dbt]

Need to communicate that this would break any redshift user who is depending on overwriting a postgres macro.

[padbk]

This change would be much appreciated.

dbt-postgres 1.8.0 is creating issues for those running Linux due to a change in Psycopg2 dependencies, see dbt-labs/dbt-postgres#96

dbt-redshift does not need Psycopg2 anymore, so removing this dependency is a much needed simplfication. 🙏

[jtcohen6]

We've identified a blocker for this work:

• Example of potential breaking change: https://gist.github.com/jtcohen6/2426ec9614828b09bc67ee7a83423517
• Change we'd like to make in dbt-core to avoid this breaking change: dbt-labs/dbt-core#10280