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)
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)