Remove new_issue_url from external issue trackers

Summary

Follow-up from !58899 (comment 549891689):

it looks like new_issues_url isn't used anymore, but I'm not completely sure.

• The line in the docs was added in gitlab-foss!16353 (merged), unfortunately without any context.
• I think we stopped using it with gitlab-foss!12130 (merged), previously we'd redirect /issues/new if an external issue tracker was configured.

After enabling the Redmine integration I still get the GitLab issue form everywhere, so at least that part seems to be gone. Also found this feature request to bring it back, which I guess confirms that 😉 gitlab-foss#47736 (closed)

The same probably applies to other external issue trackers with a new_issue_url:

• https://docs.gitlab.com/ee/user/project/integrations/bugzilla.html
• https://docs.gitlab.com/ee/user/project/integrations/ewm.html
• app/models/project_services/custom_issue_tracker_service.rb (no docs)

The one exception seems to be Jira, which still links to new_issue_url in the issue list (but it's automatically generated from the Jira base URL).

We can probably go ahead and remove the field completely, I'll open a separate issue. For this MR I'd say let's just keep it as it is currently 😅

Improvements

Remove new_issue_url from:

• app/models/project_services/issue_tracker_service.rb
• app/models/project_services/issue_tracker_data.rb
• app/models/project_services/bugzilla_service.rb
• app/models/project_services/custom_issue_tracker_service.rb
• app/models/project_services/ewm_service.rb
• app/models/project_services/redmine_service.rb

Also update the related docs, and check that we're not exposing the field in the API.

Docs pages to update:

• doc/api/services.md
  • ### Create/Edit Bugzilla service
  • ### Create/Edit Custom Issue Tracker service
  • ### Create/Edit Redmine service
• doc/user/project/integrations/bugzilla.md
• doc/user/project/integrations/custom_issue_tracker.md
• doc/user/project/integrations/ewm.md
• doc/user/project/integrations/redmine.md

Risks

• We haven't fully verified that the URL isn't used anymore, but so far it seems likely.
• JiraService also has a new_issue_url method, but it's hard-coded so it shouldn't be affected by this change.