Remove issue_trackers scope

As part of the work related to
https://gitlab.com/gitlab-org/gitlab/-/issues/218526 we have simplified
the use of issue_trackers and external_issue_trackers.

For more information see the related merge requests:
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/32330
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/33263
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/36063

app/models/service.rb

@@ -56,7 +56,7 @@ class Service < ApplicationRecord
   validates :instance, uniqueness: { scope: :type }, if: -> { instance? }
   validate :validate_is_instance_or_template
 
-  scope :issue_trackers, -> { where(category: 'issue_tracker') }
+  scope :external_issue_trackers, -> { where(category: 'issue_tracker').active }
   scope :external_wikis, -> { where(type: 'ExternalWikiService').active }
   scope :active, -> { where(active: true) }
   scope :by_type, -> (type) { where(type: type) }
@@ -76,7 +76,6 @@ class Service < ApplicationRecord
   scope :wiki_page_hooks, -> { where(wiki_page_events: true, active: true) }
   scope :deployment_hooks, -> { where(deployment_events: true, active: true) }
   scope :alert_hooks, -> { where(alert_events: true, active: true) }
-  scope :external_issue_trackers, -> { issue_trackers.active }
   scope :deployment, -> { where(category: 'deployment') }
 
   default_value_for :category, 'common'