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 <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Auto-Submit: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go