[lib] Remove any potential existing messages for threads that are no longer watched
[lib] Remove any potential existing messages for threads that are no longer watched

Summary: In the case that a thread is "unwatched", we want to remove any potential existing messages in `messageStore.messages` that correspond to the no longer watched thread

Test Plan: in progress

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D1913

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