Removes test_samples-impl ref to older virtualenv package

Author: chalmerlowe

.kokoro/test-samples-impl.sh

@@ -33,8 +33,7 @@ export PYTHONUNBUFFERED=1
 env | grep KOKORO
 
 # Install nox
-# `virtualenv==20.26.6` is added for Python 3.7 compatibility
-python3.9 -m pip install --upgrade --quiet nox virtualenv==20.26.6
+python3.9 -m pip install --upgrade --quiet nox virtualenv
 
 # Use secrets acessor service account to get secrets
 if [[ -f "${KOKORO_GFILE_DIR}/secrets_viewer_service_account.json" ]]; then