Remove redundant pipe from pdeathsig.c test.

A pipe was was left over from a development version of pdeathsig.c and
is not needed.

Process C waits for a signal that'll be generated when process B
exists. Process B waits for process D to send it a byte via pipe_db
before it exits. Process D sends the byte after it has started
ptrace()ing process C. The point of the test is to show that process C
receives the signal because process B exited, even though C has been
reparented to process D. The pipe pipe_cd isn't doing anything useful
(though in an earlier version of the patch it did). Clean that up by
removing the useless pipe.

Submitted by: Thomas Munro
MFC after: 6 days
Differential revision: https://reviews.freebsd.org/D15214