ref(discover) Remove another reference event usage (#20839)

Missed this in #20817

Author: markstory

src/sentry/discover/endpoints/discover_key_transactions.py

@@ -129,7 +129,7 @@ def get(self, request, organization):
 
         queryset = KeyTransaction.objects.filter(organization=organization, owner=request.user)
 
-        def get_event_stats(query_columns, query, params, rollup, reference_event=None):
+        def get_event_stats(query_columns, query, params, rollup):
             return key_transaction_timeseries_query(
                 selected_columns=query_columns,
                 query=query,