container: plugins: root: update example plugin

`dfi` API headers must be managed on an as-needed basis.

container/plugins/root/example/internal/example.hh

@@ -25,8 +25,6 @@
 
 #include <string>
 
-// NOTE: `dfi` API headers are included by default
-
 namespace dfi::plugin::example
 {
 //! \brief An example plugin implementation class