Add "absent" states to remove services

[jpmckinney]

For example, removing apache takes a bit of effort: #206 (comment)

The apache-formula has a "clean" state: https://github.com/saltstack-formulas/apache-formula/blob/master/docs/README.rst#apache-clean

[jpmckinney]

In the intervening years, we haven't removed services (we're more likely to rebuild the server from scratch). So, closing as not needed.