Remove old DTD for ptop files - deprecated by the new rnc version, and

having this around is confusing.

assign/ptop.dtd

-<!--
-   EMULAB-COPYRIGHT
-   Copyright (c) 2005-2006 University of Utah and the Flux Group.
-   All rights reserved.
--->
-
-<!ELEMENT ptop (node*,link*)>
-<!ATTLIST ptop testbed CDATA #REQUIRED>
-
-<!-- Nodes -->
-
-<!ELEMENT node (node_type+,node_feature*,node_option*)>
-<!ATTLIST node name ID #REQUIRED>
-
-<!ELEMENT node_type ((limit|unlimited),static?)>
-<!ATTLIST node_type name CDATA #REQUIRED>
-<!-- Include something about 'switch' or 'transit' -->
-
-<!ELEMENT static EMPTY>
-
-<!ELEMENT limit EMPTY>
-<!ATTLIST limit slots CDATA #REQUIRED>
-
-<!ELEMENT unlimited EMPTY>
-
-<!ELEMENT node_feature (feature_weight, violatable?)>
-<!ATTLIST node_feature type (normal | local-additive | global-exclusive |
-                             global-inclusitve) "normal"                     
-                       name CDATA #REQUIRED>
-<!ELEMENT feature_weight EMPTY>
-<!ATTLIST feature_weight weight CDATA #REQUIRED>
-<!ELEMENT violatable EMPTY>
-
-<!ELEMENT node_option (option_name,option_value)>
-<!-- Stronger 'typing'? -->
-<!ELEMENT option_name EMPTY>
-<!ATTLIST option_name name CDATA #REQUIRED>
-<!ELEMENT option_value EMPTY>
-<!ATTLIST option_value value CDATA #REQUIRED>
-
-<!--- Links -->
-
-<!ELEMENT link (endpoint,endpoint,delay_spec,link_type+)>
-<!ATTLIST link name ID #REQUIRED>
-
-<!ELEMENT endpoint EMPTY>
-<!ATTLIST endpoint node IDREF #REQUIRED
-                   interface CDATA #REQUIRED>
-<!ELEMENT delay_spec (bandwidth,delay,loss)>
-<!ELEMENT bandwidth EMPTY>
-<!ATTLIST bandwidth value CDATA #REQUIRED>
-<!ELEMENT delay EMPTY>
-<!ATTLIST delay value CDATA #REQUIRED>
-<!ELEMENT loss EMPTY>
-<!ATTLIST loss value CDATA #REQUIRED>
-<!ELEMENT link_type EMPTY>
-<!ATTLIST link_type type CDATA #REQUIRED>