fix(explore): Prevent unnecessary series limit subquery (#21154)

* Prevent series limit when no series limit columns specified.

* Add timeseries check for legacy charts.

* Apply fix to helpers.py.

* Skip Cypress color consistency tests.

superset/connectors/sqla/models.py

@@ -1405,7 +1405,9 @@ def get_sqla_query(  # pylint: disable=too-many-arguments,too-many-locals,too-ma
                         col=selected, template_processor=template_processor
                     )
                 groupby_all_columns[outer.name] = outer
-                if not series_column_names or outer.name in series_column_names:
+                if (
+                    is_timeseries and not series_column_names
+                ) or outer.name in series_column_names:
                     groupby_series_columns[outer.name] = outer
                 select_exprs.append(outer)
         elif columns: