[lib] Remove community from thick threads

Summary:
Thick threads can't have a community.

https://linear.app/comm/issue/ENG-8937/remove-community-from-mutablethickrawthreadinfo

More context D12914.

Test Plan: Made sure that it's possible to create a sidebar. Also checked if persisting and reading thin and thick threads from SQLite works on web.

Reviewers: kamil, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12967