internal: simplify version, delete genv

Simplify the build constraints in signal_unix.go and signal_notunix.go
to mirror the cmd/go/internal/base package.

Delete the unused genv command. It was used in the era of releasebot,
back when CLs to add golang.org/dl commands were being created by hand.
These CLs are generated by relui now.

Change-Id: Ic471394a30021c599a832f7b2a966160f2a121ed
Reviewed-on: https://go-review.googlesource.com/c/dl/+/540222
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dmitshur

internal/genv/main.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build aix || darwin || dragonfly || freebsd || js || linux || netbsd || openbsd || solaris || wasip1
+//go:build unix || js || wasip1
 
 package version