libdpkg: Remove obsolete variable references in parsedb() comment

The warnto and warncount variables were removed in commits
ba908783e306ba38457a84fd6d975ca69e351e56 and
8451065cf85cb8a17dcd17e0cd2ca1fdef7cb749 respectively.

lib/dpkg/parse.c

@@ -424,7 +424,7 @@ parse_stanza(struct parsedb_state *ps, struct field_state *fs,
 /**
  * Parse an RFC-822 style file.
  *
- * warnto, warncount and donep may be NULL.
+ * donep may be NULL.
  * If donep is not NULL only one package's information is expected.
  */
 int parsedb(const char *filename, enum parsedbflags flags,