all: cleanup unused code, part 1 (#10661)

Run `staticcheck` with `U1000` to find unused code. This cleans up about
a half of it. I'll do the other half separately to keep PRs manageable.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

version/modinfo_test.go

@@ -7,7 +7,6 @@ import (
 	"flag"
 	"os/exec"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"testing"
 
@@ -36,13 +35,6 @@ func TestFindModuleInfo(t *testing.T) {
 	}
 }
 
-func exe() string {
-	if runtime.GOOS == "windows" {
-		return ".exe"
-	}
-	return ""
-}
-
 var findModuleInfoName = flag.String("module-info-file", "", "if non-empty, test findModuleInfo against this filename")
 
 func TestFindModuleInfoManual(t *testing.T) {