crypto/x509: remove TestPlatformVerifierLegacy tests

They are no longer necessary, woohoo!

Updates #52108
Fixes #56791

Change-Id: I11a4c17162da4295309f74f2f8362bab0f506f78
Reviewed-on: https://go-review.googlesource.com/c/go/+/548976
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: rolandshoemaker