Why this change?
Backfilling can take a long time on a site with many users. As a
result, the `DistriburedMutex` will warn in the logs when the block takes
more than 60 seconds to complete. We can increase the lock validity but
the method is currently only called from a job which has a
`cluster_concurrency` set to `1`. Therefore, it is actually not
necessary for us to hold a `DistributedMutex` here.
What does this change do?
1. Removes the distributed mutex and adds a comment about the risk of
calling the `SidebarSiteSettingsBackfiller#backfill!` method.a
2. Delete old sidebar category section links in batches for all users
to avoid a single long running query.
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