Remove dbt-postgres dependency

[mikealfare]

resolves #811

Problem

dbt-redshift depends on dbt-postgres for a very small number of macros, and no python code. This is a large dependency for minimal convenience. This recently caused issues, like the psycopg2 install. It also makes it a little harder to remove dependencies on things like pg_% tables.

Solution

• remove the dependency on dbt-postgres
• vendor referenced macros into dbt-redshift
• ensure tests still complete successfully.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-redshift contributing guide.

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-redshift contributing guide.