chore: Cleanup database sessions (#10427)

Co-authored-by: John Bodley <john.bodley@airbnb.com>

superset/models/slice.py

@@ -300,7 +300,6 @@ class Slice(
         :returns: The resulting id for the imported slice
         :rtype: int
         """
-        session = db.session
         make_transient(slc_to_import)
         slc_to_import.dashboards = []
         slc_to_import.alter_params(remote_id=slc_to_import.id, import_time=import_time)
@@ -309,7 +308,6 @@ class Slice(
         slc_to_import.reset_ownership()
         params = slc_to_import.params_dict
         datasource = ConnectorRegistry.get_datasource_by_name(
-            session,
             slc_to_import.datasource_type,
             params["datasource_name"],
             params["schema"],
@@ -318,11 +316,11 @@ class Slice(
         slc_to_import.datasource_id = datasource.id  # type: ignore
         if slc_to_override:
             slc_to_override.override(slc_to_import)
-            session.flush()
+            db.session.flush()
             return slc_to_override.id
-        session.add(slc_to_import)
+        db.session.add(slc_to_import)
         logger.info("Final slice: %s", str(slc_to_import.to_json()))
-        session.flush()
+        db.session.flush()
         return slc_to_import.id
 
     @property