Convert /instances/* controller specs to request specs (#27988)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

spec/controllers/api/v2/instances_controller_spec.rb

@@ -1,22 +0,0 @@
-# frozen_string_literal: true
-
-require 'rails_helper'
-
-describe Api::V2::InstancesController do
-  render_views
-
-  let(:user)  { Fabricate(:user) }
-  let(:token) { Fabricate(:accessible_access_token, resource_owner_id: user.id) }
-
-  before do
-    allow(controller).to receive(:doorkeeper_token) { token }
-  end
-
-  describe 'GET #show' do
-    it 'returns http success' do
-      get :show
-
-      expect(response).to have_http_status(200)
-    end
-  end
-end