[schema] Deprecating the table_columns.database_expression column (#7653)

superset/connectors/base/models.py

@@ -394,7 +394,6 @@ class BaseColumn(AuditMixinNullable, ImportMixin):
             "groupby",
             "is_dttm",
             "type",
-            "database_expression",
             "python_date_format",
         )
         return {s: getattr(self, s) for s in attrs if hasattr(self, s)}