types/structs: drop unused structs package (#3981)

types/structs/structs.go

-// Copyright (c) Tailscale Inc & AUTHORS
-// SPDX-License-Identifier: BSD-3-Clause
-
-// Package structs contains the Incomparable type.
-package structs
-
-// Incomparable is a zero-width incomparable type. If added as the
-// first field in a struct, it marks that struct as not comparable
-// (can't do == or be a map key) and usually doesn't add any width to
-// the struct (unless the struct has only small fields).
-//
-// By making a struct incomparable, you can prevent misuse (prevent
-// people from using ==), but also you can shrink generated binaries,
-// as the compiler can omit equality funcs from the binary.
-type Incomparable [0]func()