fix #2262 Make the fact that optional are not in pom anymore explicit
fix #2262 Make the fact that optional are not in pom anymore explicit

The `withXml` block had become no-op, because OptionalDependenciesPlugin
doesn't generate `<dependency>` blocks for these.

It is replaced with a comment that makes it clearer that these optional
dependencies are not included in the pom anymore, on purpose.
reactor/reactor-core