Remove old JSON LD signature implementation

Only PeerTube was compatible with it, and the library has moved on
RsaSignature2018 and removed RsaSignature2017 support. We had to create
a dirty fork of the RsaSignature2017 branch, which is not ideal.

Now we use the Mastodon implementation, that most other AP
implementations that support JSONLD signatures use.

server/helpers/custom-jsonld-signature.ts

@@ -1,6 +1,5 @@
 import * as AsyncLRU from 'async-lru'
 import * as jsonld from 'jsonld'
-import * as jsig from 'jsonld-signatures'
 import { logger } from './logger'
 
 const CACHE = {
@@ -79,6 +78,4 @@ jsonld.documentLoader = (url, cb) => {
   lru.get(url, cb)
 }
 
-jsig.use('jsonld', jsonld)
-
-export { jsig, jsonld }
+export { jsonld }