[Feature] Remove dbt-postgres dependency

[mikealfare]

Describe the feature

dbt-redshift depends on dbt-postgres for 8 macros. These are all "pass-through" macros, e.g.:

{% macro redshift__create_schema(relation) -%}
  {{ postgres__create_schema(relation) }}
{% endmacro %}

We can eliminate the dependency on dbt-postgres through a combination of vendoring the dbt-postgres macros into dbt-redshift and simply relying on the default in dbt-adapters.

Tasks

• remove references to dbt-postgres macros (currently 7 instances in adapters.sql and 1 in snapshot_merge.sql)
• remove the dependency in adapters.redshift.__init__.py
• remove reference to dbt-postgres in tests (currently 1 in test_macros.py, 1 in test_query_comment.py, and 1 comment in unit/utils.py)
• remove dependency in setup.py and dev-requirements.txt
• update docs to reflect that postgres macros will no longer be available to use in custom macros; include the workaround of simply replacing postgres__ with redshift__ to get the new macro

[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: [Feature][Spike] Allow adapters to specify multiple namespaces  dbt-core#10280