math/rand/v2: remove Rand.Seed

math/rand/v2: remove Rand.Seed

Removing Rand.Seed lets us remove lockedSource as well,
along with the ambiguity in globalRand about which source
to use.

For #61716.

Change-Id: Ibe150520dd1e7dd87165eacaebe9f0c2daeaedfd
Reviewed-on: https://go-review.googlesource.com/c/go/+/502498
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Rob Pike <r@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>