[ADAP 619] Fix hardcoded file format in python materializations

[ben-schreiber]

resolves #803 docs dbt-labs/docs.getdbt.com/#

Problem

The attached bug #803

Solution

Maintain delta as the default file_format for backwards compatibility

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] 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
• [x] 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

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