DEV: Drop chat_uploads table and model and remove old references (#20926)

Followup to 0924f874bd9aca3515e17d0d084a35058e9d5757,
we migrated Chat::Upload records to UploadReference records
there and have not been making new Chat::Upload records
for some time, we can now delete the model and table.

plugins/chat/app/jobs/regular/chat/channel_delete.rb

@@ -44,7 +44,6 @@ module Jobs
 
           # if the uploads are not used anywhere else they will be deleted
           # by the CleanUpUploads job in core
-          ::DB.exec("DELETE FROM chat_uploads WHERE chat_message_id IN (#{message_ids.join(",")})")
           ::UploadReference.where(
             target_id: message_ids,
             target_type: ::Chat::Message.sti_name,

plugins/chat/app/models/chat/message.rb

@@ -60,11 +60,6 @@ module Chat
       CLASS_MAPPING.invert.fetch(self) || super
     end
 
-    # TODO (martin) Remove this when we drop the ChatUpload table
-    has_many :chat_uploads,
-             dependent: :destroy,
-             class_name: "Chat::Upload",
-             foreign_key: :chat_message_id
     has_one :chat_webhook_event,
             dependent: :destroy,
             class_name: "Chat::WebhookEvent",

plugins/chat/app/models/chat/upload.rb

@@ -1,32 +0,0 @@
-# frozen_string_literal: true
-
-# TODO (martin) DEPRECATED: Remove this model once UploadReference has been
-# in place for a couple of months, 2023-04-01
-#
-# NOTE: Do not use this model anymore, chat messages are linked to uploads via
-# the UploadReference table now, just like everything else.
-module Chat
-  class Upload < ActiveRecord::Base
-    self.table_name = "chat_uploads"
-
-    belongs_to :chat_message, class_name: "Chat::Message"
-    belongs_to :upload
-
-    deprecate *public_instance_methods(false)
-  end
-end
-
-# == Schema Information
-#
-# Table name: chat_uploads
-#
-#  id              :bigint           not null, primary key
-#  chat_message_id :integer          not null
-#  upload_id       :integer          not null
-#  created_at      :datetime         not null
-#  updated_at      :datetime         not null
-#
-# Indexes
-#
-#  index_chat_uploads_on_chat_message_id_and_upload_id  (chat_message_id,upload_id) UNIQUE
-#