Revert "disable hybrid engine unless flag set"

This reverts commit 4c968b7.

Author: TrevorBergeron

bigframes/pandas/io/api.py

@@ -279,7 +279,6 @@ def _read_gbq_colab(  # type: ignore[overload-overlap]
     *,
     pyformat_args: Optional[Dict[str, Any]] = ...,
     dry_run: Literal[False] = ...,
-    use_hybrid_engine: bool = ...,
 ) -> bigframes.dataframe.DataFrame:
     ...
 
@@ -290,7 +289,6 @@ def _read_gbq_colab(
     *,
     pyformat_args: Optional[Dict[str, Any]] = ...,
     dry_run: Literal[True] = ...,
-    use_hybrid_engine: bool = ...,
 ) -> pandas.Series:
     ...
 
@@ -300,7 +298,6 @@ def _read_gbq_colab(
     *,
     pyformat_args: Optional[Dict[str, Any]] = None,
     dry_run: bool = False,
-    use_hybrid_engine: bool = False,
 ) -> bigframes.dataframe.DataFrame | pandas.Series:
     """A Colab-specific version of read_gbq.
 
@@ -315,9 +312,7 @@ def _read_gbq_colab(
         dry_run (bool):
             If True, estimates the query results size without returning data.
             The return will be a pandas Series with query metadata.
-        use_hybrid_engine (bool):
-            If True, and session not started, new session started will use
-            hybrid execution which pushes some execution to local cpu.
+
     Returns:
         Union[bigframes.dataframe.DataFrame, pandas.Series]:
             A BigQuery DataFrame if `dry_run` is False, otherwise a pandas Series.
@@ -350,7 +345,7 @@ def _read_gbq_colab(
             dry_run=True,
         )
         _set_default_session_location_if_possible_deferred_query(create_query)
-        if use_hybrid_engine and not config.options.bigquery._session_started:
+        if not config.options.bigquery._session_started:
             config.options.bigquery.enable_polars_execution = True
 
     return global_session.with_default_session(

tests/system/small/session/test_read_gbq_colab.py

@@ -77,8 +77,7 @@ def test_read_gbq_colab_fresh_session_is_hybrid():
         GROUP BY name
         ORDER BY total DESC
         LIMIT 300
-        """,
-        use_hybrid_engine=True,
+        """
     )
     session = df._session
     executions_before_python = session._metrics.execution_count