[dev.regabi] cmd/compile: remove workarounds for go/constant issues

These were fixed in CLs 273086 and 273126, which have been merged back
into dev.regabi already.

Passes toolstash -cmp.

Change-Id: I011e9ed7062bc034496a279e21cc163267bf83fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/280643


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>