Revert "runtime: fix memclrNoHeapPointersPreemptible"

This reverts CL 756122.

Reason: Adding preemptible memclrNoHeapPointers exposes existing unsafe
use of notInHeapSlice, causing crashes. Revert the memclr stack until
the underlying issue is fixed.

For #78254.

Change-Id: I8a234d1a6dddf70d9aa5ecda1ac8bb25deb08248
Reviewed-on: https://go-review.googlesource.com/c/go/+/757342
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>