Remove unused `redirect_to_external_issue_tracker` method

Its usage has been removed at 2fa22a07.

app/controllers/projects/issues_controller.rb

@@ -257,18 +257,6 @@ class Projects::IssuesController < Projects::ApplicationController
     return render_404 unless @project.feature_available?(:issues, current_user)
   end
-  def redirect_to_external_issue_tracker
-    external = @project.external_issue_tracker
-
-    return unless external
-
-    if action_name == 'new'
-      redirect_to external.new_issue_path
-    else
-      redirect_to external.issue_tracker_path
-    end
-  end
-
   def issue_params
     params.require(:issue).permit(*issue_params_attributes)
   end