None of the two places that call lfstackUnpack use the second argument.
This simplifies a followup CL that merges the lfstack{Pack,Unpack}
implementations.

Change-Id: I3c93f6259da99e113d94f8c8027584da79c1ac2c
Reviewed-on: https://go-review.googlesource.com/21595
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>