remove helloworld from Herd

[jvasileff]

com.example.helloworld/1.0.0 is one of the templates for new (and old) users to try out. But it's also available on Herd at https://herd.ceylon-lang.org/modules/com.example.helloworld.

This is confusing. For instance, you may have a failed compile, accidentally compile for the wrong backend, or be in the wrong directory, yet run ...helloworld still works.

I know "we never remove modules from Herd." But I think this one should be an exception.