[keyserver/lib] update deleteFarcasterChannelTag to use communites table

Summary:
Now that we have a communities table that was introduced in D11903, we can use it to better improve our deleteFarcasterChannelTag method

Depends on D11975

Test Plan: Removed the farcaster channel tag and confirmed that the farcaster channel id and the blob holder were removed in the correct row of the communites table + also confirmed that the blob holding the farcaster channel <> comm community association was deleted

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: bartek, ashoat, tomek

Differential Revision: https://phab.comm.dev/D11978