os: don't store reference count in Process.state

os: don't store reference count in Process.state

We only need a reference count in processHandle.

For #70907
Fixes #71564

Change-Id: I209ded869203dea10f12b070190774fb5f1d3d71
Reviewed-on: https://go-review.googlesource.com/c/go/+/638577
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>