io/fs: remove darwin/arm64 special condition

It isn't necessary on darwin/arm64 (macOS).

It was probably leftover from the old code when darwin/arm64
meant iOS. The test passes on iOS builder. Apparently this is
not needed either. Remove.

Change-Id: I6fa0c55d6086325d4b722862c4fe6c30bcd6e6e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/280158


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>