[keyserver] delete all fc channel tags

Summary: i had tagged a bunch of communities with farcaster channels and needed a way to remove the tags in a way that not only clean up my keyserver but also clean up dynamodb and s3. this script may be helpful to others, so putting up a diff

Test Plan: successfully removed all the farcaster tags i had created from my mariadb communities table, ddb, and s3

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13638