AWSSD: Cleanup empty Service

[voro015]

What would you like to be added:
AWSSD provider aka AWS CloudMap should remove CloudMap Services when its last Instance de-registered.

Why is this needed:
In case when external-dns is used with fqdn-template to generate CloudMap Services it will create new CM Services each time, but in case when there are no Endpoints left for particular CM Service it will be left empty in CM Namespace.
If it's ran in highly scalable env at some point CM Namespace will contains tens of thousands of empty CM Services and external-dns will read all of them on each reconciliation loop.
Some form of cleanup is required to use it successfully.