runtime: remove the unused scanIdx from mspan

After CL 700496, mspan.scanIdx is never used, this CL just remove it.

Change-Id: I41ce9902957c0cfa6fbf26b66a2a7787b179376a
Reviewed-on: https://go-review.googlesource.com/c/go/+/737220
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>