runtime/pprof/internal: delete package gzip0

runtime/pprof/internal: delete package gzip0

rsc's change golang.org/cl/32455 added a mechanism
that allows pprof to depend on gzip without introducing
an import cycle. This obsoletes the need for the gzip0
package, which was created solely to remove the need
for that dependency.

Change-Id: Ifa3b98faac9b251f909b84b4da54742046c4e3ad
Reviewed-on: https://go-review.googlesource.com/33137
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>