storage_controller: don't drop observed state on unavailable detach

The observed state removal may race with the inline updates of the
observed state done from `Service::node_activate_reconcile`.

This was intended to work as follows:
1. Detaches while the node is unavailable remove the entry from the
   observed state.
2. `Service::node_activate_reconcile` diffs the locations returned
   by the pageserver with the observed state and detaches in-line
   when required.

This commit removes step (1) and lets background reconciliations
deal with the mismatch between the intent and observed state.

storage_controller/src/reconciler.rs

@@ -211,11 +211,12 @@ impl Reconciler {
         lazy: bool,
     ) -> Result<(), ReconcileError> {
         if !node.is_available() && config.mode == LocationConfigMode::Detached {
-            // Attempts to detach from offline nodes may be imitated without doing I/O: a node which is offline
-            // will get fully reconciled wrt the shard's intent state when it is reactivated, irrespective of
-            // what we put into `observed`, in [`crate::service::Service::node_activate_reconcile`]
-            tracing::info!("Node {node} is unavailable during detach: proceeding anyway, it will be detached on next activation");
-            self.observed.locations.remove(&node.get_id());
+            // [`crate::service::Service::node_activate_reconcile`] will update the observed state
+            // when the node comes back online. At that point, the intent and observed states will
+            // be mismatched and a background reconciliation will detach.
+            tracing::info!(
+                "Node {node} is unavailable during detach: proceeding anyway, it will be detached via background reconciliation"
+            );
             return Ok(());
         }