crypto/hpke: remove unused hybridKEM field

crypto/hpke: remove unused hybridKEM field

hybridKEM.pqGenerateKey is not used anywhere. Having it around
is confusing, as it suggests that the mlkem seed is generated within
the mlkem package, when it is not.

Change-Id: Ie0bdb79d6b774db7b21dfff2b966452463f1ce08
Reviewed-on: https://go-review.googlesource.com/c/go/+/726260
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>