all: cleanup unused code, part 2 (#10670)

And enable U1000 check in staticcheck.

Updates #cleanup

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

wgengine/router/router_test.go

@@ -11,6 +11,7 @@ import (
 	"tailscale.com/types/preftype"
 )
 
+//lint:ignore U1000 used in Windows/Linux tests only
 func mustCIDRs(ss ...string) []netip.Prefix {
 	var ret []netip.Prefix
 	for _, s := range ss {