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/dmail.rb

@@ -214,9 +214,6 @@ class Dmail < ApplicationRecord
   def validate_sender_is_not_banned
     if from.try(:is_banned?)
       errors[:base] << "Sender is banned and cannot send messages"
-      return false
-    else
-      return true
     end
   end