internal/runtime/maps: make clear also erase tombstones

This will make future uses of the map faster because the probe
sequences will likely be shorter.

Change-Id: If10f3af49a5feaff7d1b82337bbbfb93bcd9dcb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/633076


Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>