daemon: Remove reference to immutable.{cc,hh}, which no longer exist.

* daemon.am (libutil_a_SOURCES): Remove nix/libutil/immutable.cc.
  (libutil_headers): Remove nix/libutil/immutable.hh.  These files were
  removed in Nix commit def5160 (post 1.2).
  Reported by Nikita Karetnikov <nikita@karetnikov.org>.

daemon.am

@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2012 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
 #
 # This file is part of GNU Guix.
 #
@@ -55,14 +55,12 @@ libformat_a_CPPFLAGS =				\
 libutil_a_SOURCES =				\
   nix/libutil/archive.cc			\
   nix/libutil/serialise.cc			\
-  nix/libutil/immutable.cc			\
   nix/libutil/util.cc				\
   nix/libutil/xml-writer.cc			\
   nix/libutil/hash.cc				\
   nix/libutil/gcrypt-hash.cc
 
 libutil_headers =				\
-  nix/libutil/immutable.hh			\
   nix/libutil/hash.hh				\
   nix/libutil/serialise.hh			\
   nix/libutil/xml-writer.hh			\