refactor: still_ongoing assert is shared in all paths

pageserver/src/tenant/timeline/detach_ancestor.rs

@@ -1113,6 +1113,10 @@ pub(super) async fn detach_and_reparent(
                 .is_some_and(|b| b.blocked_by_detach_ancestor()),
         )
     };
+    assert!(
+        still_ongoing,
+        "cannot (detach? reparent)? complete if the operation is not still ongoing"
+    );
 
     let ancestor = if let Some(ancestor) = detached.ancestor_timeline.as_ref() {
         assert!(
@@ -1135,10 +1139,6 @@ pub(super) async fn detach_and_reparent(
         if let Some(ancestor) = existing {
             Ancestor::Detached(ancestor, ancestor_lsn)
         } else {
-            assert!(
-                still_ongoing,
-                "cannot complete if the operation is not still ongoing"
-            );
             let direct_children =
                 reparented_direct_children(detached, tenant).map_err(Error::from)?;
             return Ok(DetachingAndReparenting::AlreadyDone(direct_children));
@@ -1154,10 +1154,6 @@ pub(super) async fn detach_and_reparent(
     //
     // if we crash after this operation, a retry will allow reparenting the remaining timelines as
     // gc is blocked.
-    assert!(
-        still_ongoing,
-        "to detach or reparent, gc must still be blocked"
-    );
 
     let (ancestor, ancestor_lsn, was_detached) = match ancestor {
         Ancestor::NotDetached(ancestor, ancestor_lsn) => {