Remove `object_storage_cluster_join_mode='local'` — it is now equivalent to `'allow'`

[alsugiliazova]

Summary

After upstream PR ClickHouse/ClickHouse#94748 (Fix invalid result of joining two -Cluster table functions, commit 5b364db3ae0, merge 2b769e0ae56, present on antalya-26.3 but not on antalya-26.1/25.8), the analyzer unconditionally wraps the leftmost IStorageCluster table in a subquery whenever a JOIN is present. As a result, object_storage_cluster_join_mode='local' and 'allow' are now behaviorally indistinguishable.

The local-mode code path in IStorageCluster is dead code for cluster-leftmost JOINs.

Why local is now redundant

src/Planner/PlannerJoinTree.cpp:2828–2865 rewrites the query tree from

SELECT … FROM cluster_fn(...) AS t1 JOIN local AS t2 ON …

into

SELECT … FROM (SELECT <cols> FROM cluster_fn(...)) AS t1 JOIN local AS t2 ON …

before IStorageCluster::read is called. IStorageCluster then receives only the inner column-fetch subquery — no JOIN, no CROSS JOIN, no local-column WHERE. So:

• IStorageCluster::updateQueryWithJoinToSendIfNeeded (src/Storages/IStorageCluster.cpp:212–303): getQueryTreeInfo reports no join ⇒ the LOCAL branch is a no-op, identical to the ALLOW branch.
• IStorageCluster::getQueryProcessingStage (src/Storages/IStorageCluster.cpp:628–651): same — both modes fall through to the same WithMergeableState path.

The JOIN executes on the initiator regardless of mode; shards only see SELECT cols FROM cluster_fn(...).