cmd/internal/obj/ppc64: remove unused C_XER optab class

cmd/internal/obj/ppc64: remove unused C_XER optab class

This halves the size of the xcmp lookup table.

Change-Id: I543fb72709ca45c026e9b7d8084a78f2a8fcd43e
Reviewed-on: https://go-review.googlesource.com/c/go/+/542295
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Jayanth Krishnamurthy <jayanth.krishnamurthy@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>