Revert "os: make use of pidfd on linux"

Revert "os: make use of pidfd on linux"

This reverts CL 528438.

Reason for revert: Implicated in "bad FD" test failures. Full extent of
issue still unclear.

For #62654.
Fixes #65857.

Change-Id: I066e38040544c506917e90255bd0e330964a0276
Reviewed-on: https://go-review.googlesource.com/c/go/+/566477
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>