models: drop unnecessary presence validations.

In rails 5, belongs_to associations automatically validate that the
associated item is present, meaning that we don't need to validate these
things manually any more.

app/models/user_name_change_request.rb

@@ -1,6 +1,6 @@
 class UserNameChangeRequest < ApplicationRecord
   after_initialize :initialize_attributes, if: :new_record?
-  validates_presence_of :user_id, :original_name, :desired_name
+  validates_presence_of :original_name, :desired_name
   validates_inclusion_of :status, :in => %w(pending approved rejected)
   belongs_to :user
   belongs_to :approver, :class_name => "User", optional: true