Remove the iterations cadence placeholder

There is no need for the iterations cadence placeholder in FOSS
as iterations cadence mode is a EE only model that does not tie to
any FOSS functionality.

app/models/iterations/cadence.rb

-# frozen_string_literal: true
-
-# Placeholder class for model that is implemented in EE
-class Iterations::Cadence < ApplicationRecord
-  self.table_name = 'iterations_cadences'
-end
-
-Iterations::Cadence.prepend_if_ee('::EE::Iterations::Cadence')

ee/app/models/ee/iterations/cadence.rb

-# frozen_string_literal: true
-
-module EE
-  module Iterations
-    module Cadence
-      extend ActiveSupport::Concern
-
-      prepended do
-        belongs_to :group
-        has_many :iterations, foreign_key: :iterations_cadence_id, inverse_of: :iterations_cadence
-
-        validates :title, presence: true
-        validates :start_date, presence: true
-        validates :group_id, presence: true
-        validates :active, presence: true
-        validates :automatic, presence: true
-      end
-    end
-  end
-end

ee/app/models/iterations/cadence.rb

+# frozen_string_literal: true
+
+module Iterations
+  class Cadence < ApplicationRecord
+    self.table_name = 'iterations_cadences'
+
+    belongs_to :group
+    has_many :iterations, foreign_key: :iterations_cadence_id, inverse_of: :iterations_cadence
+
+    validates :title, presence: true
+    validates :start_date, presence: true
+    validates :group_id, presence: true
+    validates :active, presence: true
+    validates :automatic, presence: true
+  end
+end