runtime/metrics: remove unused StopTheWorld Description field

This change removes the as-of-yet unused StopTheWorld field in the
Description struct. Adding a new field to a struct is much easier than
removing it, so let's save it for when we actually need it.

Change-Id: I8074b8569187c1a148500575fa8a661534e875d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/282632


Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Austin Clements <austin@google.com>