[Flow202][lib][skip-ci] [13/23] Fix Flow issues relating to lastPruned that was accidentally left in message-reducer
Summary:
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 D9704
Test Plan: Confirm the Flow errors go away
Reviewers: atul
Reviewed By: atul
Subscribers: tomek, wyilio
Differential Revision: https://phab.comm.dev/D9705