runtime: remove unused scanSize parameter to gcmarknewobject

runtime: remove unused scanSize parameter to gcmarknewobject

This was left over from the old pacer, and never removed when the old
pacer was removed in Go 1.19.

Change-Id: I79e5f0420c6100c66bd06129a68f5bbab7c1ea8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/429256
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>