giflib_5_1: remove the alias (appears unused)

In 3605f128 I somehow forgot about the 5.1 names that now don't make
sense anymore.  I can't see any evidence of _5_1 alias ever being used
or (potentially) useful; _5 might make sense, but for now I couldn't
see why add it.

pkgs/development/libraries/giflib/default.nix

@@ -0,0 +1,24 @@
+{stdenv, fetchurl, xmlto, docbook_xml_dtd_412, docbook_xsl, libxml2 }:
+
+stdenv.mkDerivation rec {
+  name = "giflib-5.2.1";
+  src = fetchurl {
+    url = "mirror://sourceforge/giflib/${name}.tar.gz";
+    sha256 = "1gbrg03z1b6rlrvjyc6d41bc8j1bsr7rm8206gb1apscyii5bnii";
+  };
+
+  postPatch = ''
+    substituteInPlace Makefile \
+      --replace 'PREFIX = /usr/local' 'PREFIX = ${builtins.placeholder "out"}'
+  '';
+
+  buildInputs = [ xmlto docbook_xml_dtd_412 docbook_xsl libxml2 ];
+
+  meta = {
+    description = "A library for reading and writing gif images";
+    platforms = stdenv.lib.platforms.unix;
+    license = stdenv.lib.licenses.mit;
+    maintainers = with stdenv.lib.maintainers; [ fuuzetsu ];
+    branch = "5.2";
+  };
+}