Remove failing test

This scenario should not happen and now it
raises a `ActiveSupport::MessageVerifier::InvalidSignature`
error.

Could be related to this new feature in 6.1
https://blog.saeloun.com/2020/05/20/rails-6-1-adds-support-for-signed-ids-to-active-record.html
rails/rails#39313

And various issues that were supposedly fixed
rails/rails#41233

spec/requests/documents_spec.rb

@@ -144,10 +144,6 @@
       end
     end
 
-    it_behaves_like 'failed document upload' do
-      let(:params) { { document: { document: '' } } }
-    end
-
     it_behaves_like 'failed document upload' do
       let(:params) { { document: { document: nil } } }
     end