Remove cross-db modification exception in Clusters::Applications::Runner

Managed cluster applications are no longer available, so we can remove
this logic entirely.

app/models/clusters/applications/runner.rb

@@ -41,9 +41,7 @@ module Clusters
       end
 
       def prepare_uninstall
-        ::Gitlab::Database::QueryAnalyzers::PreventCrossDatabaseModification.allow_cross_database_modification_within_transaction(url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/350180') do
-          runner&.update!(active: false)
-        end
+        # No op, see https://gitlab.com/gitlab-org/gitlab/-/issues/350180.
       end
 
       def post_uninstall

spec/models/clusters/applications/runner_spec.rb

@@ -101,19 +101,6 @@ RSpec.describe Clusters::Applications::Runner do
     end
   end
 
-  describe '#prepare_uninstall' do
-    it 'pauses associated runner' do
-      active_runner = create(:ci_runner, contacted_at: 1.second.ago)
-
-      expect(active_runner.active).to be_truthy
-
-      application_runner = create(:clusters_applications_runner, :scheduled, runner: active_runner)
-      application_runner.prepare_uninstall
-
-      expect(active_runner.active).to be_falsey
-    end
-  end
-
   describe '#make_uninstalling!' do
     subject { create(:clusters_applications_runner, :scheduled, runner: ci_runner) }