Don't require customer when creating a billing group

The customer field will not be required after NUC-7501 is fixed.

src/main/java/com/brkt/client/BillingGroupRequestBuilder.java

@@ -15,7 +15,7 @@ public class BillingGroupRequestBuilder extends RequestBuilder {
 
     private BillingGroupRequestBuilder(boolean forCreate) {
         if (forCreate) {
-            reqBuilder.requiredFields("customer", "name");
+            reqBuilder.requiredFields("name");
         }
     }