cmd/compile: remove more dead code and data structures

Remove more now-redundant code, methods, and types
associated with transition to register ABI.
Repaired some broken comments.

Tested on link-register architectures (arm64, ppc64le)

Updates #40724.

Change-Id: Ie8433f6d38ec4a1d9705f22dcb596f267d81f203
Reviewed-on: https://go-review.googlesource.com/c/go/+/304189


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>