Remove unused invalid_insert_overwrite_delta_msg variable

[dbeatty10]

resolves #962

Problem

While researching dbt-labs/docs.getdbt.com#4639, it looks like this code is unused since #700.

Solution

Searching both dbt-spark and dbt-databricks didn't yield any other references to invalid_insert_overwrite_delta_msg:

• https://github.com/search?q=repo%3Adbt-labs%2Fdbt-spark%20invalid_insert_overwrite_delta_msg&type=code
• https://github.com/search?q=repo%3Adatabricks%2Fdbt-databricks%20invalid_insert_overwrite_delta_msg&type=code

So it appears safe to remove it altogether.

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
• 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)

[mikealfare]

If we're touching this, there's a part of me that really wants to move this entire macro into a python method which gets called in the macro. It's all logic and no templating. But I'm fine approving for the sake of brevity in changes.