cmd/compile: remove residual register GC map code

cmd/compile: remove residual register GC map code

We used to generate register GC maps as an experimental approach
for asynchronous preemption, which later we chose not to take.
Most of the register GC map code are already removed. One
exception is that the ssa.Register type still contains a field
for the register map index. Remove it.

Change-Id: Ib177ebce9548aa5ffbcaedd4b507240ea7df8afe
Reviewed-on: https://go-review.googlesource.com/c/go/+/651076
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>