crypto/x509/internal/macos: remove unused types and functions

crypto/x509/internal/macos: remove unused types and functions

Some of the types and functions in the macos package are unused since
CL 353132. They can be removed.

Change-Id: Ifb7c9619d3c77b83852e785b82877dfa3ca8fe6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/643277
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>