providers/docker: flatten stack so it runs at one point

plugins/providers/docker/action.rb

@@ -110,9 +110,8 @@ module VagrantPlugins
 
             b2.use Call, IsBuild do |env2, b3|
               if env2[:result]
-                b3.use EnvSet, force_confirm_destroy: true,
-                  skip_docker_host_machine_sync_folders_disable: true
-                b3.use action_destroy
+                b3.use EnvSet, force_confirm_destroy: true
+                b3.use action_destroy.flatten
               end
             end
 

plugins/providers/docker/action/host_machine_sync_folders_disable.rb

@@ -15,7 +15,6 @@ module VagrantPlugins
         end
 
         def call(env)
-          return @app.call(env) if env[:skip_docker_host_machine_sync_folders_disable]
           return @app.call(env) if !env[:machine].provider.host_vm?
 
           # Read our random ID for this instance