[lib] Get rid of fetchFollowedFarcasterChannelsWithFilter

Summary:
We don't actually use this at all, except for the single callsite from fetchFollowedFarcasterChannels, which doesn't need the filter.

If we need it again in the future, it should be trivial to reintroduce.

Test Plan: Flow

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13974