Refactor ProjectTreeRestorer logic

This rewrites a majority of the logic
for tree restorer to make it much cleaner
in terms of relations between objects
and make it much easier to persist objects
at very explicit points.

app/models/project.rb

@@ -1962,27 +1962,6 @@ class Project < ApplicationRecord
     (auto_devops || build_auto_devops)&.predefined_variables
   end
 
-  def append_or_update_attribute(name, value)
-    if Project.reflect_on_association(name).try(:macro) == :has_many
-      # if this is 1-to-N relation, update the parent object
-      value.each do |item|
-        item.update!(
-          Project.reflect_on_association(name).foreign_key => id)
-      end
-
-      # force to drop relation cache
-      public_send(name).reset # rubocop:disable GitlabSecurity/PublicSend
-
-      # succeeded
-      true
-    else
-      # if this is another relation or attribute, update just object
-      update_attribute(name, value)
-    end
-  rescue ActiveRecord::RecordInvalid => e
-    raise e, "Failed to set #{name}: #{e.message}"
-  end
-
   # Tries to set repository as read_only, checking for existing Git transfers in progress beforehand
   #
   # @return [Boolean] true when set to read_only or false when an existing git transfer is in progress