Remove old JSON LD signature implementation

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.