pargzip: remove outdated package doc sentence
pargzip: remove outdated package doc sentence

We stopped using the system gzip some time ago
and removed the option in golang.org/cl/29432

Change-Id: I6f41af16357a5b29bbf337c4820409c78a75fff8
Reviewed-on: https://go-review.googlesource.com/36907
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build