crypto/x509: remove leftover CertificateRequest field

Removes the KeyUsage field that was missed in the rollback in
CL 281235.
Also updates CreateCertificateRequest to reflect that these fields
were removed.

For #43407.
Updates #43477.
Updates #37172.

Change-Id: I6244aed4a3ef3c2460c38af5511e5c2e82546179
Reviewed-on: https://go-review.googlesource.com/c/go/+/287392


Trust: Alexander Rakoczy <alex@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>