Dead code.

This field is left over from Go 1.4, when we elided the fake write
barrier in this case. Today, it's unused (always false).
The upcoming append/slice changes handle this case again,
but without needing this field.

Change-Id: Ic6f160b64efdc1bbed02097ee03050f8cd0ab1b8
Reviewed-on: https://go-review.googlesource.com/9789Reviewed-by: David Chase <drchase@google.com>