[lib] Use channel instead of channel/search Neynar endpoint

Summary:
We're exceeding our compute unit allotment with Neynar: ENG-9930.

It's mostly the channel/search API. I'm guessing it's more expensive than the channel API, which has fewer capabilities.

Turns out we actually already have NeynarClient.fetchFarcasterChannelInfo, which calls the channel API. But we don't have any callsites to it anymore.

This diff replaces the code for NeynarClient.fetchFarcasterChannelByID with NeynarClient.fetchFarcasterChannelInfo, and subsequently removes the latter.

Depends on D13978

Test Plan: Tested that channel avatars still load on both web and native

Reviewers: varun, will

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13979