spanconfig: consider deleting `spanconfig.store.fallback_config_override`

It isn't supported or marked public, but if some a bot or someone found it and messed with it, it could cause some odd/confusing to debug behaviors given it is not commonly used or tested. It might be safer just not having this laying around at all.

(if we do this: we could also delete `RegisterProtobufSetting` and associated code that is used solely for this setting)

Jira issue: CRDB-64513

Search the repository for `spanconfig.store.fallback_config_override` and trace the associated `RegisterProtobufSetting` code. Confirm whether the setting and registration code are used elsewhere, then remove only the unused pieces and run the relevant spanconfig tests to verify the database still builds and passes its checks.

Written by the indexing model from the issue text.