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The purpose of `maybe_fix_legacy_secondary_index_mv_schema` was to deal with legacy materialized view schemas used for secondary indexes, schemas which were created before the notion of "computed columns" was introduced. Back then, secondary index schemas would use a regular "token" column. Later it became a computed column and old schemas would be migrated during rolling upgrade. The migration code was introduced in 2019 (db8d4a0cc6) and then fixed in 2020 (d473bc9b06). The fix was present in Enterprise 2022.1 and in OSS 4.5. So, assuming that users don't try crazy things like upgrading from 2021.X to 2023.X (which we do not support), all clusters will have already executed the migration code once they upgrade to 2023.X, meaning we can get rid of it. The main motivation of this patch is to get rid of the `db::schema_tables::merge_schema` call in `parse_schema_tables`. In Raft mode this was the only call to `merge_schema` outside "group 0 code" and in fact it is unsafe -- it uses locally generated mutations with locally generated timestamp (`api::new_timestamp()`), so if we actually did it, we would permanently diverge the group 0 state machine across nodes (the schema pulling code is disabled in Raft mode). Fortunately, this should be dead code by now, as explained in the previous paragraph. The migration code is now turned into a sanity check, if the users try something crazy, they will get an error instead of silent data corruption.