cmd/dist: delete special case for release branches without VERSION

cmd/dist: delete special case for release branches without VERSION

findgoversion has some logic from before the go1 release that only
has effect when on a release branch without a VERSION file.

Starting with release-branch.go1 and the go1 tag a decade ago,
release branch have always had a VERSION file checked in.
(The commit that adds/updates the VERSION file is what is tagged.)

Since we have no need to support old branches like release-branch.r60,
and such scenarios don't come up in modern Go, delete it to simplify
this code a bit. Should the VERSION file situation change, we'd need
to rework this code anyway.

Fixes #42345.

Change-Id: I13f27babd37aaa5cec30fefde1b8e6ccce816461
Reviewed-on: https://go-review.googlesource.com/c/go/+/393954
Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>