crypto/ecdsa: improve benchmarks

crypto/ecdsa: improve benchmarks

While at it, drop P-224 benchmarks. Nobody cares about P-224.

Change-Id: I31db6fedde6026deff36de963690275dacf5fda1
Reviewed-on: https://go-review.googlesource.com/c/go/+/451196
Auto-Submit: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>