This reverts commit I44d8dd8020bc61b9c75e0566c34ca9cc56b23f9d.

Reason for revert: Broke the build. (Fixes #17352)

Original change's description:
> x/sys/unix: add *linkat functions on linux
> 
> * expose linkat, readlinkat, symlinkat
> 
> Fixes golang/go#14459
> 
> Change-Id: I44d8dd8020bc61b9c75e0566c34ca9cc56b23f9d
> Reviewed-on: https://go-review.googlesource.com/20129
> Reviewed-by: Russ Cox <rsc@golang.org>
> 

Change-Id: Ic114a5e60b0caef82c21123156963b2a8232d9d4
Reviewed-on: https://go-review.googlesource.com/30491
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>