Fix replicator class list

In development and test environments, the models which include
`ReplicableModel` may not be loaded yet when
`ReplicableModel.replicators` is called, therefore causing `Replicator`
classes to be missing from the list.

Hard code the list for now, while we take time to implement it properly.

ee/app/workers/geo/registry_sync_worker.rb

@@ -78,7 +78,7 @@ module Geo
     end
 
     def replicator_classes
-      Gitlab::Geo::ReplicableModel.replicators
+      Gitlab::Geo.replicator_classes
     end
   end
 end

ee/lib/gitlab/geo.rb

@@ -157,5 +157,12 @@ module Gitlab
 
       _(template) % { url: url }
     end
+
+    # TODO: Avoid having to maintain a list. Discussions related to possible
+    # solutions can be found at
+    # https://gitlab.com/gitlab-org/gitlab/-/issues/227693
+    def self.replicator_classes
+      [::Geo::PackageFileReplicator]
+    end
   end
 end

ee/lib/gitlab/geo/replicable_model.rb

@@ -25,8 +25,6 @@ module Gitlab
         def with_replicator(klass)
           raise ArgumentError, 'Must be a class inheriting from Gitlab::Geo::Replicator' unless klass < ::Gitlab::Geo::Replicator
 
-          Gitlab::Geo::ReplicableModel.add_replicator(klass)
-
           class_eval <<-RUBY, __FILE__, __LINE__ + 1
             define_method :replicator do
               @_replicator ||= klass.new(model_record: self)
@@ -39,16 +37,6 @@ module Gitlab
         end
       end
 
-      def self.add_replicator(klass)
-        @_replicators ||= []
-        @_replicators << klass
-      end
-
-      def self.replicators
-        @_replicators ||= []
-        @_replicators.filter { |replicator| const_defined?(replicator.to_s) }
-      end
-
       # Geo Replicator
       #
       # @abstract

ee/spec/lib/gitlab/geo_spec.rb

@@ -335,4 +335,13 @@ RSpec.describe Gitlab::Geo, :geo, :request_store do
       expect(described_class.interacting_with_primary_message(url)).to eq(message)
     end
   end
+
+  describe '.replicator_classes' do
+    it 'returns an Array of replicator classes' do
+      result = described_class.replicator_classes
+
+      expect(result).to be_an(Array)
+      expect(result).to include(Geo::PackageFileReplicator)
+    end
+  end
 end