[lib] Remove `removeMessagesFromUnwatchedThreadOps` from `mergeNewMessages`
[lib] Remove `removeMessagesFromUnwatchedThreadOps` from `mergeNewMessages`

Summary: We no longer need these ops because we're calling `filterByNewThreadInfos` which creates them for us.

Test Plan: set breakpoint and observed expected behavior

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D1928

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