Reverting revisions 30103-30106: "always set nixpkgs.config.{state,store}Dir", etc.

After the change from revision 30103, nixos-rebuild suddenly consumed
freaky amounts of memory. I had to abort the process after it had
allocated well in excess of 30GB(!) of RAM. I'm not sure what is causing
this behavior, but undoing that assignment fixes the problem. The other
two commits needed to be revoked, too, because they depend on 30103.

svn path=/nixos/trunk/; revision=30127

modules/services/web-servers/apache-httpd/default.nix

@@ -339,7 +339,7 @@ let
     # But do allow access to files in the store so that we don't have
     # to generate <Directory> clauses for every generated file that we
     # want to serve.
-    <Directory ${config.nixpkgs.config.nix.storeDir}>
+    <Directory /nix/store>
         Order allow,deny
         Allow from all
     </Directory>