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/gflags/default.nix

@@ -1,19 +0,0 @@
-{ stdenv, fetchurl, cmake }:
-
-stdenv.mkDerivation
-  { name = "gflags-2.2.1";
-    src = fetchurl
-      { url = "https://github.com/gflags/gflags/archive/v2.2.1.tar.gz";
-        sha256 = "03lxc2ah8i392kh1naq99iip34k4fpv22kwflyx3byd2ssycs9xf";
-      };
-    nativeBuildInputs = [ cmake ];
-    # for case-insensitive filesystems
-    prePatch = "mv BUILD BUILD.bazel";
-
-    meta = with stdenv.lib; {
-      description = "C++ library that implements commandline flags processing";
-      homepage = "https://github.com/gflags/gflags";
-      license = licenses.bsd3;
-      platforms = platforms.unix;
-    };
-  }