Drop microbatch environment variable from tests

[QMalcolm]

resolves #N/A

Problem

Microbatch functionality is no longer gated behind an environment variable. There is now a behavior flag, however, setting the behavior flag is only necessary if one has a custom microbatch model in their project. Core handles and tests this behavior. We should still run the tests we're running for microbatch in dbt-bigquery, but no longer need to specify the environment variable

Solution

Drop the microbatch environment variable from the tests

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
• This PR includes tests, or 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) or this PR has already received feedback and approval from Product or DX

[github-actions]

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