Remove references to old python validator in rules.html

[jcpitre]

Describe the problem

We still have this in the generated rules.html file:

It's time to get rid of it.

Suggest a fix or an alternative

This is set up when using the GtfsValidationNotice. e.g. for DuplicateKeyNotice we have:

@GtfsValidationNotice(
    severity = ERROR,
    sections = @SectionRefs(FILE_REQUIREMENTS),
    urls = {
      @UrlRef(
          label = "Original Python validator implementation",
          url = "https://github.com/google/transitfeed")
    })
public class DuplicateKeyNotice extends ValidationNotice { ...

We need to do a simple search and remove this UrlRef everywhere it's used.