[web] Simplify return value of changeThreadSettingsAction

Summary: Noticed this when reading through @tomek's feedback on D4416 when requesting review on D4931. Looks like I forgot to add this in after he accepted!

Test Plan: Flow, @tomek's feedback

Reviewers: tomek, atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D4932