gc.state.placeholder was added in 5a6e511c but never used.

Change-Id: I5a621507279d5bb1f3991b7a412d9a63039c464e
Reviewed-on: https://go-review.googlesource.com/38755Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>