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/pg_similarity.nix

@@ -13,7 +13,6 @@ stdenv.mkDerivation {
   buildInputs = [ postgresql gcc ];
   buildPhase = "USE_PGXS=1 make";
   installPhase = ''
-    mkdir -p $out/bin   # for buildEnv to setup proper symlinks
     install -D pg_similarity.so -t $out/lib/
     install -D ./{pg_similarity--unpackaged--1.0.sql,pg_similarity--1.0.sql,pg_similarity.control} -t $out/share/extension
   '';