Remove unused link_to_all method from UserReferenceFilter

What does this MR do and why?

Removes the dead Banzai::Filter::References::UserReferenceFilter#link_to_all.

The method's only caller was removed in Disable @all mention, remove feature flag (!232918 - merged); ideally that MR would've removed the dead code. We're doing it now!

Screenshots or screen recordings

n/a; only removing dead code.

How to set up and validate locally

Since we're only removing an unreferenced method ...

• Do a repo-wide grep for link_to_all on master: there's only its definition.
• Check all CI passes.
• Read !232918 (merged)'s diff to see how it used to be used to feel sure there's no other likely magical metaprogramming use which somehow escaped CI, coverage, etc.

MR acceptance checklist

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