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>

tstest/archtest/archtest_test.go

@@ -14,7 +14,7 @@ import (
 func TestAlignedAtomicInt64(t *testing.T) {
 	type T struct {
 		A atomicbitops.Int64
-		x int32
+		_ int32
 		B atomicbitops.Int64
 	}