postgresql: always create the $out/bin directory

This is needed because some PostgreSQL plugins don't have a bin
directory. If only these plugins are listed in cfg.extraPlugins buildEnv
will turn $out/bin into a symbolic link to ${pg}/bin. Lateron we try to
rm $out/bin/{pg_config,postgres,pg_ctl} which will then fail because
$out/bin will be read-only.

pkgs/servers/sql/postgresql/ext/pgroonga.nix

@@ -15,7 +15,6 @@ stdenv.mkDerivation rec {
   makeFlags = [ "HAVE_MSGPACK=1" ];
 
   installPhase = ''
-    mkdir -p $out/bin
     install -D pgroonga.so -t $out/lib/
     install -D ./{pgroonga-*.sql,pgroonga.control} -t $out/share/extension
   '';