Remove a KASSERT I added in 1.73 to catch uninitialized pipes.

It must be removed because it is done without the pipe being locked
via pipelock() and therefore is vulnerable to races with pipespace()
erroneously triggering it by temporarily zero'ing out the structure
backing the pipe.

It looks as if this assertion is not needed because all manipulation
of the data changed by pipespace() _is_ protected by pipelock().

Reported by: kris, mckusick

sys/kern/sys_pipe.c

@@ -927,8 +927,6 @@ pipe_write(fp, uio, active_cred, flags, td)
 		return(error);
 	}
 		
-	KASSERT(wpipe->pipe_buffer.buffer != NULL, ("pipe buffer gone"));
-
 	orig_resid = uio->uio_resid;
 
 	while (uio->uio_resid) {