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/user_password_reset_nonce.rb

@@ -15,7 +15,6 @@ class UserPasswordResetNonce < ApplicationRecord
   def validate_existence_of_email
     if !User.with_email(email).exists?
       errors[:email] << "is invalid"
-      return false
     end
   end