Revert "runtime: only poll network from one P at a time in findRunnable"

This reverts commit 352dd2d9.

Reason for revert: cockroachdb benchmark failing. Likely due to CL 564197.

For #73474

Change-Id: Id5d83cd8bb8fe9ee7fddb8dc01f1a01f2d40154e
Reviewed-on: https://go-review.googlesource.com/c/go/+/667336


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>