crypto/x509: remove bad generate statement

crypto/x509: remove bad generate statement

CL488855 added a broken go:generate statement, which referenced a
file which didn't exist. Remove the statement.

The generator is in the commit message for CL488855, if it is ever
actually needed.

Change-Id: I6b18d5b4f38f82b27681b9b758e9642543aa6e43
Reviewed-on: https://go-review.googlesource.com/c/go/+/565435
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>