Retire old "xxx_namespace" declarations from the IDL

While poking into the IDL syntax, I found that hidden gem:

| tok_xsd_namespace tok_literal
    {
      pwarning(1, "'xsd_namespace' is deprecated. Use 'namespace xsd' instead");
      pdebug("Header -> tok_xsd_namespace tok_literal");
      declare_valid_program_doctext();
      if (g_parse_mode == PROGRAM) {
        g_program->set_namespace("cocoa", $2);   // what ... COCOA ?!?
      }
    }

While thinking whether I should do sth about it, I came to the conclusion that the time ist right to "Get rid of this" since today probably really "everyone is using the new hotness", as the comments say.

Opinions?

If there are no objections, I'll provide a patch to convert the warnings into an error and remove the rest of it.