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/unfollow_service_spec.rb

@@ -30,13 +30,6 @@ RSpec.describe UnfollowService, type: :service do
     it 'destroys the following relation' do
       expect(sender.following?(bob)).to be false
     end
-
-    it 'sends an unfollow salmon slap' do
-      expect(a_request(:post, "http://salmon.example.com/").with { |req|
-        xml = OStatus2::Salmon.new.unpack(req.body)
-        xml.match(OStatus::TagManager::VERBS[:unfollow])
-      }).to have_been_made.once
-    end
   end
 
   describe 'remote ActivityPub' do