[lib] Avoid directly mutating MessageStore in messageStorePruneActionType
[lib] Avoid directly mutating MessageStore in messageStorePruneActionType

Summary: I noticed that this code was directly mutating the objects in the `MessageStore` instead of treating them as immutable. This makes it impossible for selectors to notice changes and update components that are watching the `MessageStore`, and thus is considered bad practice. This diff updates the code to avoid mutating the objects directly.

Test Plan: Flow, careful reading

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D915

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal