Replace prometheus service with a more general policy

This change was first made
in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40088 when
I needed to add a policy for another service. Noting that the
prometheus service thus became redundant, it is removed.

app/policies/prometheus_service_policy.rb → app/policies/service_policy.rb

 # frozen_string_literal: true
 
-class PrometheusServicePolicy < ::BasePolicy
-  delegate { @subject.project }
+class ServicePolicy < BasePolicy
+  delegate(:project)
 end

spec/policies/prometheus_service_policy_spec.rb

@@ -2,12 +2,12 @@
 
 require 'spec_helper'
 
-RSpec.describe PrometheusServicePolicy, :models do
+RSpec.describe PrometheusService, :models do
   let(:integration) { create(:prometheus_service) }
   let(:project) { integration.project }
   let(:user) { create(:user) }
 
-  subject(:policy) { described_class.new(user, integration) }
+  subject(:policy) { Ability.policy_for(user, integration) }
 
   describe 'rules' do
     it { is_expected.to be_disallowed :admin_project }