runtime: for fallback hash, get rid of constant xors

runtime: for fallback hash, get rid of constant xors

There's no need for these. If hashkey[i] is uniform over [0,1<<64), then
hashkey[0]^C has exactly the same distribution, for any constant C.

Change-Id: I4e10c27eff15e4b7a45139654ac41f410b1b12fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/580218
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>