os: remove unused variable in unix implementation of File.readdir

Change-Id: I0dd8a325bce6ed12d1ec1dc206ded62398925aef
Reviewed-on: https://go-review.googlesource.com/c/go/+/267758


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>