Delete config properties

As an application developer, deployer or tester or a framework developer or tester, I expect to be able to specify that a configuration property should be removed in the event that it would otherwise have a value, so that I can easily make necessary adjustments to the configuration at deployment or testing time.

Based on https://github.com/eclipse-ee4j/config/discussions/27#discussioncomment-1809272 with some modifications

Start by reading the issue and the linked Eclipse Config discussion to understand the proposed semantics for removing configuration properties. Identify the relevant configuration API and implementation entry points, then define the expected behavior and tests before attempting the feature.

Written by the indexing model from the issue text.