Remove JIRA spec for ExternalIssueReferenceFilter

These specs do not exist in CE and don't add anything useful themselves.
There's no application code that initializes a JiraIssue instance and it
currently behaves exactly the same as a regular ExternalIssue.

spec/lib/banzai/filter/external_issue_reference_filter_spec.rb

@@ -73,13 +73,5 @@ describe Banzai::Filter::ExternalIssueReferenceFilter, lib: true do
 
       expect(link).to eq helper.url_for_issue("#{reference}", project, only_path: true)
     end
-
-    it 'adds to the results hash' do
-      ext = JiraIssue.new(reference, project)
-
-      result = reference_pipeline_result("Issue #{reference}")
-      expect(result[:references][:external_issue]).not_to be_empty
-      expect(result[:references][:external_issue]).to eq [ext]
-    end
   end
 end