Remove unused `IssuesHelper#merge_requests_sentence`

Its last usage has been removed at eb3e7bdf.

app/helpers/issues_helper.rb

@@ -86,14 +86,6 @@ module IssuesHelper
     return 'hidden' if issue.closed? == closed
   end
 
-  def merge_requests_sentence(merge_requests)
-    # Sorting based on the `!123` or `group/project!123` reference will sort
-    # local merge requests first.
-    merge_requests.map do |merge_request|
-      merge_request.to_reference(@project)
-    end.sort.to_sentence(last_word_connector: ', or ')
-  end
-
   def confidential_icon(issue)
     icon('eye-slash') if issue.confidential?
   end

spec/helpers/issues_helper_spec.rb

@@ -58,16 +58,6 @@ describe IssuesHelper do
     end
   end
 
-  describe "merge_requests_sentence" do
-    subject { merge_requests_sentence(merge_requests)}
-    let(:merge_requests) do
-      [build(:merge_request, iid: 1), build(:merge_request, iid: 2),
-       build(:merge_request, iid: 3)]
-    end
-
-    it { is_expected.to eq("!1, !2, or !3") }
-  end
-
   describe '#award_user_list' do
     it "returns a comma-separated list of the first X users" do
       user = build_stubbed(:user, name: 'Joe')