appease RubocopRails: Rails/RedundantForeignKey

> Specifying the default value for foreign_key is redundant.

Apparently I didn't have to specify these foreign keys back when
RubocopRails barked about missing inverse relationships?

Signed-off-by: Robb Kidd <rkidd@chef.io>

src/supermarket/app/models/cookbook.rb

@@ -95,7 +95,7 @@ class Cookbook < ApplicationRecord
   belongs_to :category, optional: true
   belongs_to :owner, class_name: "User", foreign_key: :user_id, inverse_of: :owned_cookbooks
   has_one :chef_account, through: :owner
-  belongs_to :replacement, class_name: "Cookbook", foreign_key: :replacement_id, inverse_of: :replaces, optional: true
+  belongs_to :replacement, class_name: "Cookbook", inverse_of: :replaces, optional: true
   has_many :replaces, class_name: "Cookbook", foreign_key: :replacement_id, inverse_of: :replacement, dependent: :nullify
   has_many :collaborators, as: :resourceable, inverse_of: :resourceable # rubocop:disable Rails/HasManyOrHasOneDependent
   has_many :collaborator_users, through: :collaborators, source: :user

src/supermarket/app/models/user.rb

@@ -7,7 +7,7 @@ class User < ApplicationRecord
   # Associations
   # --------------------
   has_many :accounts
-  has_many :owned_cookbooks, class_name: "Cookbook", foreign_key: "user_id", inverse_of: :owner
+  has_many :owned_cookbooks, class_name: "Cookbook", inverse_of: :owner
   has_many :cookbook_versions
   has_many :collaborators
   has_many :cookbook_followers