[release-branch.go1.13] crypto/ecdsa: remove s390x assembly

This is a revert of CL 174437 and equivalent to CL 201360.

The size of the params block passed into the KDSA instruction is
incorrect and this appears to result in out-of-bounds writes
that cause a panic in the crypto/x509 tests when run on a machine
that supports KDSA.

Remove this assembly for now. We can revisit the use of the KDSA
instruction in a future release.

Fixes #34928.

Change-Id: I7ad2fe9714b47ad04abc25f18aa235b9d2aef062
Reviewed-on: https://go-review.googlesource.com/c/go/+/201361


Run-TryBot: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>