gflags: alias to google-gflags

Both packages referred to the same repo (`gflags/gflags` at GitHub),
both packages used almost the same version (2.2.1 and 2.2.0). Therefore
I think that it's just fair to drop one of the packages to avoid
duplication and reduce the maintenance overload.

I decided to drop `gflags` originally packaged by @shlevy as it didn't
contain testing support as `google-gflags` does. The package set refers
from `gflags` to `google-gflags`, so both package names remain (to avoid
any confusions), however the `gflags` expression is removed.

pkgs/development/libraries/google-gflags/default.nix

@@ -2,13 +2,13 @@
 
 stdenv.mkDerivation rec {
   name = "google-gflags-${version}";
-  version = "2.2.0";
+  version = "2.2.1";
 
   src = fetchFromGitHub {
     owner = "gflags";
     repo = "gflags";
     rev = "v${version}";
-    sha256 = "1y5808ky8qhjwv1nf134czz0h2p2faqvjhxa9zxf8mg8hn4ns9wp";
+    sha256 = "12wkihc4f07qmhyqk3cjylj8v5xz2bjrq75p7aq1vvvj60fbp58k";
   };
 
   nativeBuildInputs = [ cmake ];