Remove Salmon and PubSubHubbub (#11205)

* Remove Salmon and PubSubHubbub endpoints

* Add error when trying to follow OStatus accounts

* Fix new accounts not being created in ResolveAccountService

spec/services/process_mentions_service_spec.rb

@@ -18,10 +18,6 @@ RSpec.describe ProcessMentionsService, type: :service do
     it 'creates a mention' do
       expect(remote_user.mentions.where(status: status).count).to eq 1
     end
-
-    it 'posts to remote user\'s Salmon end point' do
-      expect(a_request(:post, remote_user.salmon_url)).to have_been_made.once
-    end
   end
 
   context 'OStatus with private toot' do