Remove unneeded code in default snapshot materialization

[jtcohen6]

resolves #4742

Not just unneeded, also incorrect!

Description

See #4742 (comment) for the archaeology

tl;dr:

• We added this in-materialization create_schema a long time ago (Dec 2017), back when snapshots were archives defined in dbt_project.yml
• When we converted archives to snapshots (real DAG nodes), in 2019, we did the thing that makes schema creation happen implicitly during the snapshot task, just like for models in the run task
• We never removed the explicit create_schema from the snapshot materialization. We even tweaked it to fix unexpected behavior, when the real fix would have been to remove it entirely
• Let's remove it entirely :)

Testing

This is already covered by test_cross_schema_snapshot, which would fail if dbt did not first create {test_schema}_snapshotted while running the snapshot task.

Checklist

• I have signed the CLA
• 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
• I have added information about my change to be included in the CHANGELOG.