Merge branch 'remove-gon-variable' into 'master'

Unused gon variable with very bad performance

## What does this MR do?

Remove an unused variable with a lot of performance penalty. It was opening a transaction just to return a constant string.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added

See merge request !4969

CHANGELOG

@@ -13,6 +13,7 @@ v 8.10.0 (unreleased)
   - Fix changing issue state columns in milestone view
   - Fix user creation with stronger minimum password requirements !4054 (nathan-pmt)
   - Check for conflicts with existing Project's wiki path when creating a new project.
+  - Remove unused front-end variable -> default_issues_tracker
   - Add API endpoint for a group issues !4520 (mahcsig)
   - Allow [ci skip] to be in any case and allow [skip ci]. !4785 (simon_w)
 

lib/gitlab/gon_helper.rb

@@ -3,7 +3,6 @@ module Gitlab
     def add_gon_variables
       gon.api_version            = API::API.version
       gon.default_avatar_url     = URI::join(Gitlab.config.gitlab.url, ActionController::Base.helpers.image_path('no_avatar.png')).to_s
-      gon.default_issues_tracker = Project.new.default_issue_tracker.to_param
       gon.max_file_size          = current_application_settings.max_attachment_size
       gon.relative_url_root      = Gitlab.config.gitlab.relative_url_root
       gon.shortcuts_path         = help_shortcuts_path