runtime: remove GODEBUG=runtimecontentionstacks

Go 1.22 promised to remove the setting in a future release once the
semantics of runtime-internal lock contention matched that of
sync.Mutex. That work is done, remove the setting.

Previously reviewed as https://go.dev/cl/585639.

For #66999

Change-Id: I9fe62558ba0ac12824874a0bb1b41efeb7c0853f
Reviewed-on: https://go-review.googlesource.com/c/go/+/668995


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>