Remove vestigial method from MergeRequestsFinder

This isn't called from anywhere.

app/finders/merge_requests_finder.rb

@@ -66,10 +66,6 @@ class MergeRequestsFinder < IssuableFinder
     items.where(target_branch: target_branch)
   end
 
-  def item_project_ids(items)
-    items&.reorder(nil)&.select(:target_project_id)
-  end
-
   def by_wip(items)
     if params[:wip] == 'yes'
       items.where(wip_match(items.arel_table))