Table: field override footers now a union of reducers since 12.4

### What happened? Setting a field override footer calculation in TableNG no longer overrides the panel level footer calculation, but instead becomes a union of the panel and override reducers. In https://github.com/grafana/grafana/pull/102948 the first example given there for the `year` clearly shows the expected behaviour, so I wonder if this is not intentional behaviour and some later work on the multiple reducers unintentionally changed behaviour? I traced it through to https://github.com/grafana/grafana/blob/d1729c53a7f44e2e58947eb44eb896c2fb1c30b3/packages/grafana-data/src/field/fieldOverrides.ts#L355-L363 where it somewhat reads like only multiple overrides should be merged, but it's also including the panel config. ### What did you expect to happen? I'd expect a field override footer should replace panel-level footer entirely; if you want additional fields then simply include those from the panel config in the override. As-is, being unable to exclude certain panel-level reducers from specific fields is not ideal... ### Did this work before? Yes, though obviously prior to 12.4 there was only a single reducer anyway. ### How do we reproduce it? 1. Add a panel-level table footer calculation 2. Add a field override table footer calculation 3. Displayed footer contains a union of the reducer calculations rather than just those from 2 ### Is the bug inside a dashboard panel? _No response_ ### Environment (with versions)? Grafana: 12.4.0 OS: Linux Browser: Firefox ### Grafana platform? Other ### Datasource(s)? _No response_