Merge branch 'remove_unnecessary_jira_issue_class' into 'master'

Remove unused JiraIssue class and replace references with ExternalIssue

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/739

This MR can be considered as a fix of wrong CE->EE merge (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4659)

See merge request !545

app/models/jira_issue.rb

-class JiraIssue < ExternalIssue
-end

spec/helpers/merge_request_helper_spec.rb

@@ -31,9 +31,9 @@ describe MergeRequestsHelper do
     context 'for JIRA issues' do
       let(:issues) do
         [
-          JiraIssue.new('JIRA-123', project),
-          JiraIssue.new('JIRA-456', project),
-          JiraIssue.new('FOOBAR-7890', project)
+          ExternalIssue.new('JIRA-123', project),
+          ExternalIssue.new('JIRA-456', project),
+          ExternalIssue.new('FOOBAR-7890', project)
         ]
       end