fix(dbapi): allow rows to be fetched from scripts

[tswast]

The is_dml logic is not needed now that we moved to getQueryResults instead of tabledata.list (#375).

Previously, the destination table of a DML query would return a non-null value that was unreadable or would return nonsense with DML (and some DDL) queries.

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Towards #377 🦕

[shollyman]

Nice!

[mistercrunch]

[mistercrunch]

Big thanks for making this happen, this enabled us to ship this: apache/superset#11904

We're planning more work around creating a great experience around BigQuery in Superset, looking forward to more collaboration!