perf(ingest): remove unused variable in Looker ingestion

[jameslamb]

I've been looking at opportunities to contribute on metadata ingestion, so I ran pylint metadata-ingestion/src/datahub today.

This PR proposes fixing one of the warnings raised by pylint.

metadata-ingestion/src/datahub/ingestion/source/looker.py:163:16: W0612: Unused variable 'expression' (unused-variable)

This PR proposes removing that variable, which I think should make the Looker ingestion code a tiny tiny tiny bit faster and, more importantly, eliminates the risk of that code breaking if the key "expression" doesn't show up in the parsed fields.

Thanks for your time and consideration.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[shirshanka]

LGTM!