crypto/x509: drop the cgo implementation of root_darwin_amd64.go

This code was preserved just to do side-by-side testing while
transitioning to the Go implementation. There haven't been mismatch
issues, so drop the cgo code, which was making it hard to improve the Go
code without diverging.

Change-Id: I2a23039c31a46e88b94250aafbc98d4ea8daf22f
Reviewed-on: https://go-review.googlesource.com/c/go/+/232397


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>