gnu: libpng: Incorporate grafted patch.

* gnu/packages/image.scm (libpng)[replacement]: Remove field.
[source]: Add patch 'libpng-CVE-2016-10087.patch'.
(libpng-1.2)[replacement]: Remove field.
(libpng/fixed): Remove variable.

gnu/packages/image.scm

@@ -65,7 +65,6 @@
 (define-public libpng
   (package
    (name "libpng")
-   (replacement libpng/fixed)
    (version "1.6.25")
    (source (origin
             (method url-fetch)
@@ -76,6 +75,7 @@
                        (string-append
                         "ftp://ftp.simplesystems.org/pub/libpng/png/src"
                         "/libpng15/libpng-" version ".tar.xz")))
+            (patches (search-patches "libpng-CVE-2016-10087.patch"))
             (sha256
              (base32 "04c8inn745hw25wz2dc5vll5n5d2gsndj01i4srwzgz8861qvzh9"))))
    (build-system gnu-build-system)
@@ -90,18 +90,9 @@ library.  It supports almost all PNG features and is extensible.")
    (license license:zlib)
    (home-page "http://www.libpng.org/pub/png/libpng.html")))
 
-(define libpng/fixed
-  (package
-    (inherit libpng)
-    (source
-      (origin
-        (inherit (package-source libpng))
-        (patches (search-patches "libpng-CVE-2016-10087.patch"))))))
-
 (define-public libpng-1.2
   (package
     (inherit libpng)
-    (replacement #f)
     (version "1.2.57")
     (source
      (origin