[CT-827] Use truncate for seeds

[jtcohen6]

I couldn't remember why we don't use truncate for seeds... and then I remembered we've had this PR open for some time: #182

So, still possibly something to do with not working properly if the seed is an external table (location set). But this is definitely supported:

• https://spark.apache.org/docs/latest/sql-ref-syntax-ddl-truncate-table.html
• https://docs.databricks.com/sql/language-manual/sql-ref-syntax-ddl-truncate-table.html

a.k.a. the default behavior for reset_csv_table

This would allow us to support the intended behavior around --full-refresh for seeds on Spark/Databricks, too. Let's give it a go and see what happens.

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