remove IP pool rules when deleting an IP pool

app/models/ip_pool.rb

@@ -24,6 +24,7 @@ class IPPool < ApplicationRecord
   has_many :servers, :dependent => :restrict_with_exception
   has_many :organization_ip_pools, :dependent => :destroy
   has_many :organizations, :through => :organization_ip_pools
+  has_many :ip_pool_rules, :dependent => :destroy
 
   def self.default
     where(:default => true).order(:id).first