validations: drop superfluous return statements.

Returning true or false in a validation callback doesn't do anything, so
drop these superfluous return statements.

app/models/post_disapproval.rb

@@ -1,7 +1,7 @@
 class PostDisapproval < ApplicationRecord
   DELETION_THRESHOLD = 1.month
 
-  belongs_to :post, required: true
+  belongs_to :post
   belongs_to :user
   after_initialize :initialize_attributes, if: :new_record?
   validates_uniqueness_of :post_id, :scope => [:user_id], :message => "have already hidden this post"