build: remove no longer used spawn-pipe files from POTFILES.in

This should have been part of the previous commit v8.22-14-gf18999b

* po/POTFILES.in: Remove gnulib files no longer used.
Prompted by the continuous integration build failure at:
http://hydra.nixos.org/build/8288209/

po/POTFILES.in

@@ -20,14 +20,11 @@ lib/root-dev-ino.h
 lib/rpmatch.c
 lib/set-acl.c
 lib/siglist.h
-lib/spawn-pipe.c
 lib/strsignal.c
 lib/unicodeio.c
 lib/userspec.c
 lib/verror.c
 lib/version-etc.c
-lib/w32spawn.h
-lib/wait-process.c
 lib/xalloc-die.c
 lib/xfreopen.c
 lib/xmemcoll.c