chore(ci): drop inert SQLALCHEMY_WARN_20 flag from unit-test CI (#43495)

Co-authored-by: Superset Dev <dev@superset.apache.org>

.github/workflows/superset-python-unittest.yml

@@ -53,12 +53,6 @@ jobs:
         python-version: ${{ github.event_name == 'pull_request' && fromJSON('["current"]') || fromJSON('["current", "next"]') }}
     env:
       PYTHONPATH: ${{ github.workspace }}
-      # Promotes the SQLAlchemy 2.0 deprecation warnings already locked in as
-      # errors via pytest.ini's `filterwarnings` to actually run in CI, so a
-      # regression on those fails the build instead of relying on a
-      # contributor remembering to set this locally. See the migration
-      # battleplan: https://github.com/apache/superset/discussions/40273
-      SQLALCHEMY_WARN_20: "1"
     steps:
       - name: "Checkout ${{ github.ref }} ( ${{ github.sha }} )"
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1