Git history shows that after 60da4c583a84db6f08c483e366ff9b6294988160, this collection was no longer mutated.
As such, we can simplify the code by removing it.
NOTE: CI will fail on this diff. I considered the possibility of fixing Flow errors BEFORE upgrading Flow, but it wasn't possible... in some cases, the fixes to support the new version of Flow caused errors in the old version. I could have hidden these type errors with $FlowFixMe lines and then later revert those, but that seemed like too much busy work.
Depends on D10031