Remove unused cluster runner creation code causing cross-modification

As discovered at
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/74735#note_740614507
this code causes a CrossDatabaseModificationAcrossUnsupportedTablesError
since it writes to ci and non-ci tables in the context of a single
transaction. You can read more at
https://docs.gitlab.com/ee/development/database/multiple_databases.html#removing-cross-database-transactions .

This code is apparently not used and not needed anymore as we don't
allow people to install runners this way anymore. For now I decided to
just focus on removing the small amount of code that was causing
problems but it seems there could be a deeper rabbit hole here of
removing lots of unused code but I decided not to tackle that all at
once.

app/models/clusters/applications/runner.rb

@@ -50,34 +50,6 @@ module Clusters
 
       private
 
-      def ensure_runner
-        runner || create_and_assign_runner
-      end
-
-      def create_and_assign_runner
-        transaction do
-          Ci::Runner.create!(runner_create_params).tap do |runner|
-            update!(runner_id: runner.id)
-          end
-        end
-      end
-
-      def runner_create_params
-        attributes = {
-          name: 'kubernetes-cluster',
-          runner_type: cluster.cluster_type,
-          tag_list: %w[kubernetes cluster]
-        }
-
-        if cluster.group_type?
-          attributes[:runner_namespaces] = [::Ci::RunnerNamespace.new(namespace: group)]
-        elsif cluster.project_type?
-          attributes[:runner_projects] = [::Ci::RunnerProject.new(project: project)]
-        end
-
-        attributes
-      end
-
       def gitlab_url
         Gitlab::Routing.url_helpers.root_url(only_path: false)
       end
@@ -85,7 +57,6 @@ module Clusters
       def specification
         {
           "gitlabUrl" => gitlab_url,
-          "runnerToken" => ensure_runner.token,
           "runners" => { "privileged" => privileged }
         }
       end

spec/models/clusters/applications/runner_spec.rb

@@ -69,66 +69,9 @@ RSpec.describe Clusters::Applications::Runner do
       expect(values).to include('privileged: true')
       expect(values).to include('image: ubuntu:16.04')
       expect(values).to include('resources')
-      expect(values).to match(/runnerToken: ['"]?#{Regexp.escape(ci_runner.token)}/)
       expect(values).to match(/gitlabUrl: ['"]?#{Regexp.escape(Gitlab::Routing.url_helpers.root_url)}/)
     end
 
-    context 'without a runner' do
-      let(:application) { create(:clusters_applications_runner, runner: nil, cluster: cluster) }
-      let(:runner) { application.runner }
-
-      shared_examples 'runner creation' do
-        it 'creates a runner' do
-          expect { subject }.to change { Ci::Runner.count }.by(1)
-        end
-
-        it 'uses the new runner token' do
-          expect(values).to match(/runnerToken: '?#{Regexp.escape(runner.token)}/)
-        end
-      end
-
-      context 'project cluster' do
-        let(:project) { create(:project) }
-        let(:cluster) { create(:cluster, :with_installed_helm, projects: [project]) }
-
-        include_examples 'runner creation'
-
-        it 'creates a project runner' do
-          subject
-
-          runner_projects = Project.where(id: runner.runner_projects.pluck(:project_id))
-          expect(runner).to be_project_type
-          expect(runner_projects).to match_array [project]
-        end
-      end
-
-      context 'group cluster' do
-        let(:group) { create(:group) }
-        let(:cluster) { create(:cluster, :with_installed_helm, cluster_type: :group_type, groups: [group]) }
-
-        include_examples 'runner creation'
-
-        it 'creates a group runner' do
-          subject
-
-          expect(runner).to be_group_type
-          expect(runner.runner_namespaces.pluck(:namespace_id)).to match_array [group.id]
-        end
-      end
-
-      context 'instance cluster' do
-        let(:cluster) { create(:cluster, :with_installed_helm, :instance) }
-
-        include_examples 'runner creation'
-
-        it 'creates an instance runner' do
-          subject
-
-          expect(runner).to be_instance_type
-        end
-      end
-    end
-
     context 'with duplicated values on vendor/runner/values.yaml' do
       let(:stub_values) do
         {