Drop "quoted" remnants from JSON Schema & DTD data models (#672)

Author: eemeli

spec/data-model/message.dtd

@@ -29,7 +29,6 @@
 )>
 
 <!ELEMENT literal (#PCDATA)>
-<!ATTLIST literal quoted (true | false) #REQUIRED>
 
 <!ELEMENT variable (EMPTY)>
 <!ATTLIST variable name NMTOKEN #REQUIRED>

spec/data-model/message.json

@@ -9,10 +9,9 @@
       "type": "object",
       "properties": {
         "type": { "const": "literal" },
-        "quoted": { "type": "boolean" },
         "value": { "type": "string" }
       },
-      "required": ["type", "quoted", "value"]
+      "required": ["type", "value"]
     },
     "variable": {
       "type": "object",