Inline valid_state? method in DeploymentCancel

- In response to PR feedback

[cloudfoundry#1513]

Authored-by: Greg Cobb <gcobb@pivotal.io>

Author: Gerg

app/actions/deployment_cancel.rb

@@ -11,7 +11,7 @@ class << self
       def cancel(deployment:, user_audit_info:)
         deployment.db.transaction do
           deployment.lock!
-          reject_invalid_state!(deployment) unless valid_state?(deployment)
+          reject_invalid_state!(deployment) unless deployment.cancelable?
 
           begin
             AppAssignDroplet.new(user_audit_info).assign(deployment.app, deployment.previous_droplet)
@@ -29,10 +29,6 @@ def cancel(deployment:, user_audit_info:)
 
       private
 
-      def valid_state?(deployment)
-        deployment.cancelable?
-      end
-
       def reject_invalid_state!(deployment)
         raise InvalidStatus.new("Cannot cancel a deployment with status: #{deployment.status_value} and reason: #{deployment.status_reason}")
       end