runtime: clean up old markrootSpans

This change removes the old markrootSpans implementation and deletes the
feature flag.

Updates #37487.

Change-Id: Idb5a2559abcc3be5a7da6f2ccce1a86e1d7634e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/221183


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