Clean up #deleted-channels

[SparrowBrain]

If a channel is deleted, gerald fails to fetch it.

Sep  9 01:25:40 ip-10-0-0-166 web: loading subs from s3
Sep  9 01:25:41 ip-10-0-0-166 web: loaded channel ids: xxx755,xxx117,xxx883,xxx005
Sep  9 01:25:41 ip-10-0-0-166 web: DiscordAPIError: Unknown Channel
Sep  9 01:25:41 ip-10-0-0-166 web: at RequestHandler.execute (/var/app/current/node_modules/discord.js/src/rest/RequestHandler.js:154:13)
Sep  9 01:25:41 ip-10-0-0-166 web: at processTicksAndRejections (internal/process/task_queues.js:97:5)
Sep  9 01:25:41 ip-10-0-0-166 web: at async RequestHandler.push (/var/app/current/node_modules/discord.js/src/rest/RequestHandler.js:39:14)
Sep  9 01:25:41 ip-10-0-0-166 web: at async ChannelManager.fetch (/var/app/current/node_modules/discord.js/src/managers/ChannelManager.js:91:18) {
Sep  9 01:25:41 ip-10-0-0-166 web: method: 'get',
Sep  9 01:25:41 ip-10-0-0-166 web: path: '/channels/xxx005',
Sep  9 01:25:41 ip-10-0-0-166 web: code: 10003,
Sep  9 01:25:41 ip-10-0-0-166 web: httpStatus: 404
Sep  9 01:25:41 ip-10-0-0-166 web: }
Sep  9 01:25:41 ip-10-0-0-166 web: Error while fetching channel xxx005.
Sep  9 01:25:41 ip-10-0-0-166 web: subscription added to xxx755
Sep  9 01:25:41 ip-10-0-0-166 web: subscription added to xxx117
Sep  9 01:25:41 ip-10-0-0-166 web: subscription added to xxx883

Typing into discord wtf is <#xxx005> gives this: