[lib] Disable chat mentioning

Summary:
A temporary solution to a performance issue that simply disables chat mentioning by returning an empty index.

https://linear.app/comm/issue/ENG-5224/ashoats-js-thread-freezes-on-app-start-on-build-268

Test Plan: Check if chats don't appear in the tooltip.

Reviewers: kamil, inka, atul, ashoat

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9449