pargzip: remove Writer.UseSystemGzip

Go's gzip is faster than using a child process nowadays.

Also, this wasn't safe on Windows. The Windows buildlets hadn't been
deployed in over a year. When I deployed a new version (which included
the pargzip package), they started failing to gzip snapshots, breaking
test sharding. Trybots succeeded, but only after ~970 seconds, running
all the tests on the original make.bat machine.

Change-Id: I1d144b82e40285fee0b3019a27ee0ce2f3844d5a
Reviewed-on: https://go-review.googlesource.com/29432
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
2 files changed