Destroy the OAuth application when secondary becomes a primary

When a secondary becomes a primary, the OAuth application is
disassociated from that Geo node. This change destroys the OAuth
application in addition to the disassociation.

changelogs/unreleased/10225-detroy-oauth-application-when-secondary-becomes-primary.yml

+---
+title: Destroy the OAuth application when Geo secondary becomes a primary
+merge_request: 25154
+author: briankabiro
+type: other

ee/app/models/geo_node.rb

@@ -318,7 +318,9 @@ class GeoNode < ApplicationRecord
 
   def update_dependents_attributes
     if self.primary?
+      self.oauth_application&.destroy
       self.oauth_application = nil
+
       update_clone_url
     else
       update_oauth_application!

ee/spec/models/geo_node_spec.rb

@@ -186,12 +186,12 @@ describe GeoNode, :request_store, :geo, type: :model do
         end
 
         context 'when it has an oauth_application' do
-          # TODO Should it instead be destroyed?
-          # https://gitlab.com/gitlab-org/gitlab/issues/10225
-          it 'disassociates the oauth_application' do
+          it 'destroys the oauth_application' do
             primary_node.oauth_application = create(:oauth_application)
 
+            expect do
               expect(primary_node).to be_valid
+            end.to change(Doorkeeper::Application, :count).by(-1)
 
             expect(primary_node.oauth_application).to be_nil
           end