cmd/compile: remove conditional code dealing with two export formats

This removes some scaffolding introduced pre-1.7, introduced to
fix an export format bug, and to minimize conflicts with older
formats. The currently deployed and recognized format is "v1",
so don't worry about other versions. This is a step towards a
better scheme for internal export format versioning.

For #16244.

Change-Id: Ic7cf99dd2a24ad5484cc54aed44fa09332c2cf72
Reviewed-on: https://go-review.googlesource.com/27205


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>