Merge branch '13312-remove-issuable-is_assigned' into 'master'

Remove unused Issuable#is_assigned? method

Fixes #13312.

See merge request !4440

app/models/concerns/issuable.rb

@@ -171,10 +171,6 @@ module Issuable
     today? && created_at == updated_at
   end
 
-  def is_assigned?
-    !!assignee_id
-  end
-
   def is_being_reassigned?
     assignee_id_changed?
   end