libcap: remove attr dependency

libcap hasn't used attr since commit 85f38a5 ("Drop use of libattr for
our trivial use case of kernel API.").

Because it was propagated, we have to be careful about packages that
depended on it indirectly.  To mitigate this, I've tested building
every direct dependency of libcap.  The only one I found that was
missing an attr dependency of its own was bfs.

pkgs/os-specific/linux/libcap/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, lib, buildPackages, fetchurl, attr, runtimeShell
+{ stdenv, lib, buildPackages, fetchurl, runtimeShell
 , usePam ? !isStatic, pam ? null
 , isStatic ? stdenv.hostPlatform.isStatic
 
@@ -33,8 +33,6 @@ stdenv.mkDerivation rec {
 
   buildInputs = lib.optional usePam pam;
 
-  propagatedBuildInputs = [ attr ];
-
   makeFlags = [
     "lib=lib"
     "PAM_CAP=${if usePam then "yes" else "no"}"