Move innards of cluster method to environment

Environment already has a memoized deployment plaform.

No need to check for respond_to?(:cluster) now that all deployment
platforms are Clusters::Cluster.

app/models/deployment.rb

@@ -85,11 +85,7 @@ class Deployment < ApplicationRecord
   end
 
   def cluster
-    platform = project.deployment_platform(environment: environment.name)
-
-    if platform.present? && platform.respond_to?(:cluster)
-      platform.cluster
-    end
+    environment.deployment_platform&.cluster
   end
 
   def execute_hooks