crypto/internal/fips140/nistec: remove dead s390x p256OrdMul

crypto/internal/fips140/nistec: remove dead s390x p256OrdMul

The p256_ordinv.go file is build tagged to only amd64 and arm64, and
there is no p256OrdSqr.

Also, none of it will matter after CL 669895.

Change-Id: I6a6a465653c18673e3a2c7f41d2e4ac54915b365
Reviewed-on: https://go-review.googlesource.com/c/go/+/675195
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>