Revert "crypto/internal/nistec: refactor scalar multiplication"

Revert "crypto/internal/nistec: refactor scalar multiplication"

This reverts CL 471256, except for its new tests, which are expanded to
cover the case in #60717.

Updates #60717

Change-Id: I712bbcd05bf3ea4a2c9aecc9e0f02841b21aadfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/502477
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>