crypto/internal/fips/nistec: remove P-256 from generate.go

crypto/internal/fips/nistec: remove P-256 from generate.go

It's not been autogenerated since CL 627937.

Change-Id: I2d2e249c2bab673342ee42a48582b082c6958cdd
Reviewed-on: https://go-review.googlesource.com/c/go/+/629836
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>