Remove unused IssuesHelper#unassigned_filter

This helper method is not used since
https://gitlab.com/gitlab-org/gitlab/-/commit/646fbe928

No other usages in the codebase.

Code coverage also confirms that it's not used.

app/helpers/issues_helper.rb

@@ -9,13 +9,6 @@ module IssuesHelper
     classes.join(' ')
   end
 
-  # Returns an OpenStruct object suitable for use by <tt>options_from_collection_for_select</tt>
-  # to allow filtering issues by an unassigned User or Milestone
-  def unassigned_filter
-    # Milestone uses :title, Issue uses :name
-    OpenStruct.new(id: 0, title: 'None (backlog)', name: 'Unassigned')
-  end
-
   def url_for_issue(issue_iid, project = @project, options = {})
     return '' if project.nil?