DTD frontmatter in xmldsig-core-schema.xsd

[MarkDavidson]

There is some DTD frontmatter in xmldsig-core-schema.xsd. It may be unecessary. Let's see if the DTD frontmatter can be safely deleted.

The frontmatter looks like this:

<!DOCTYPE schema
  PUBLIC "-//W3C//DTD XMLSchema 200102//EN" "http://www.w3.org/2001/XMLSchema.dtd"
 [
   <!ATTLIST schema 
     xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#">
   <!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'>
   <!ENTITY % p ''>
   <!ENTITY % s ''>
  ]>

This seems to be causing some issues for people using this schema as a source for their validation using other technology stacks. The XML DSIG schema is not hosted with the TAXII Specifications, so there's NOT an analagous problem on taxii.mitre.org.