parser: Remove unnecessary dict checks when freeing strings

The following strings are never allocated from a dict:

- xmlParserCtxt.version
- xmlParserCtxt.encoding
- xmlParserCtxt.extSubURI
- xmlParserCtxt.extSubSystem
- xmlDoc.version
- xmlDoc.encoding
- xmlDoc.URL
- xmlDTD.ExternalID
- xmlDTD.SystemID
- xmlID.value

Also make the struct members point to non-const chars to avoid casts
when freeing.

include/libxml/parser.h

@@ -276,13 +276,13 @@ struct _xmlParserCtxt {
      *
      * the XML version string
      */
-    const xmlChar *version;
+    xmlChar *version;
     /**
      * @deprecated Use xmlCtxtGetDeclaredEncoding()
      *
      * the declared encoding, if any
      */
-    const xmlChar *encoding;
+    xmlChar *encoding;
     /**
      * @deprecated Use xmlCtxtGetStandalone()
      *