Stop creating GitlabIssueTrackerService records

I'm working towards removing the GitlabIssueTrackerService
class. In order to do so, I want to delete all records
of type GitlabIssueTrackerService in the services table.
The migration to remove those will follow in a later
iteration. This commit should make sure no new records
will be created.

app/models/project.rb

@@ -1199,14 +1199,6 @@ class Project < ApplicationRecord
     get_issue(issue_id)
   end
 
-  def default_issue_tracker
-    gitlab_issue_tracker_service || create_gitlab_issue_tracker_service
-  end
-
-  def issues_tracker
-    external_issue_tracker || default_issue_tracker
-  end
-
   def external_issue_reference_pattern
     external_issue_tracker.class.reference_pattern(only_long: issues_enabled?)
   end

spec/lib/banzai/filter/external_issue_reference_filter_spec.rb

@@ -52,7 +52,7 @@ describe Banzai::Filter::ExternalIssueReferenceFilter do
 
     it 'includes a title attribute' do
       doc = filter("Issue #{reference}")
-      expect(doc.css('a').first.attr('title')).to include("Issue in #{project.issues_tracker.title}")
+      expect(doc.css('a').first.attr('title')).to include("Issue in #{project.external_issue_tracker.title}")
     end
 
     it 'escapes the title attribute' do