Remove billing from the list of AWS light metricsets (elastic#21164)

On tests, loading any metricset from the AWS module is trying
to load the billing metricset as light metricset, what fails. This
shouldn't happen after elastic#15011, but it is probably happening
because on tests, not all metricsets are registered.

billing metricset was refactored to a native implementation
recently, in elastic#20527.

By now, remove billing from the list so tests can be executed.

x-pack/metricbeat/module/aws/module.yml

@@ -3,7 +3,6 @@ metricsets:
   - elb
   - ebs
   - usage
-  - billing
   - sns
   - lambda
   - dynamodb