remove support for sockets in pseudofiles

(1) we can't make them on a real filesystem except by running
something that calls socket()
(2) whyever would we want to?

tests/pseudofiles/fixture.nix

@@ -17,7 +17,6 @@
       s6-linux-init-shutdownd = dir {
         fifo = {
           type = "i";
-          subtype = "f";
           mode = "0600";
         };
         run = {
@@ -31,7 +30,6 @@
       s6-svscan-log = dir {
         fifo = {
           type = "i";
-          subtype = "f";
           mode = "0600";
         };
         notification-fd = { file = "3"; };