Cygwin: FIFO: Revert "take ownership on exec"

This reverts commit 39a9cd9465.

There is no need to explicitly take ownership in fixup_after_exec; if
ownership transfer is needed, it will be taken care of by
fhandler_fifo::close when the parent closes.  Moreover, closing the
parent's fifo_reader_thread can cause problems, such as the one
reported here:

  https://cygwin.com/pipermail/cygwin-patches/2020q2/010235.html

winsup/cygwin/fhandler.h

@@ -1443,7 +1443,7 @@ class fhandler_fifo: public fhandler_base
   { return shmem->get_shared_fc_handler_committed (); }
   void set_shared_fc_handler_committed (size_t n)
   { shmem->set_shared_fc_handler_committed (n); }
-  int update_my_handlers (bool from_exec = false);
+  int update_my_handlers ();
   int update_shared_handlers ();
   bool shared_fc_handler_updated () const
   { return shmem->shared_fc_handler_updated (); }