Drop temp table if exists

[joshuao11]

The insert overwrite does this, so let's copy that pattern here to keep the schemas/datasets tidy in BQ for the users

resolves ##1389

Problem

When using a merge strategy with the model checking the schema (To add or replace columns):

There is nothing to delete the temp table created:

Solution

We add in a step to delete and tidy up the temporary table, in the same way that is currently there for the insert_overwrite strategy:

I also checked that there was no impact on the 'ignore' strategy for on_schema_change:

There is no temporary table made here so we do not expect the DROP statement to appear:

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
  I ran tox -p and everything seems to pass
• This PR includes tests, or tests are not required/relevant for this PR

This change does change a macro but i'm unsure how to get feedback for Product or DX

• 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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