cmd/compile: delete register maps, completely

cmd/compile: delete register maps, completely

Remove go115ReduceLiveness feature gating flag, along with code
that only needed when go115ReduceLiveness is false.

Change-Id: I7571913cc74cbd17b330a0ee0160fefc9eeee66e
Reviewed-on: https://go-review.googlesource.com/c/go/+/264338
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>