crypto/ed25519: remove s390x KDSA implementation

This reverts CL 202578 and CL 230677 which added an optimization
to use KDSA when available on s390x.

Inconsistencies have been found between the two implementations
in their handling of certain edge cases. Since the Go 1.15 release
is extremely soon it seems prudent to remove this optimization
for now and revisit it in a future release.

Fixes #40475.

Change-Id: Ifb2ed9b9e573784df57383671f1c29d8abae90d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/245497


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ruixin(Peter) Bao <ruixin.bao@ibm.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>