crypto/internal/mlkem768: remove crypto/rand.Read error checking

crypto/internal/mlkem768: remove crypto/rand.Read error checking

After #66821 crypto/rand.Read can't return an error.

Change-Id: I185063a25ef70986448f2a300e5578de17f6e61e
Reviewed-on: https://go-review.googlesource.com/c/go/+/621979
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>