Revert "cmd/tailscaled: split package main into main shim + package"

This reverts commit b81bd8025b.

Not needed. See:

https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module

cmd/tailscaled/tailscaled_notwindows.go

@@ -0,0 +1,15 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !windows
+
+package main // import "tailscale.com/cmd/tailscaled"
+
+import "tailscale.com/logpolicy"
+
+func isWindowsService() bool { return false }
+
+func runWindowsService(pol *logpolicy.Policy) error { panic("unreachable") }
+
+func beWindowsSubprocess() bool { return false }