cmd/compile/internal/dwarfgen: remove unversion

cmd/compile/internal/dwarfgen: remove unversion

The unified export data format doesn't rely on embedding version
numbers in local variable names anymore, so there's no need to look
for them.

While here, simplify the checking for "~r" or "~b" to just "~",
because the next commit is going to eliminate "~b", but introduce
"~p".

Change-Id: I3ac73150ee561c66356a0c4aee5290b44a4893ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/527695
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>