btrfs-progs: remove LDSHARED

This should no longer be needed with python3 because it's now
set correctly in the interpreter.

Note that the build currently fails with and without this change.

pkgs/tools/filesystems/btrfs-progs/default.nix

@@ -20,10 +20,6 @@ stdenv.mkDerivation rec {
 
   # for python cross-compiling
   _PYTHON_HOST_PLATFORM = stdenv.hostPlatform.config;
-  # The i686 case is a quick hack; I don't know what's wrong.
-  postConfigure = stdenv.lib.optionalString (!stdenv.isi686) ''
-    export LDSHARED="$LD -shared"
-  '';
 
   # gcc bug with -O1 on ARM with gcc 4.8
   # This should be fine on all platforms so apply universally