all: depend on zstd unconditionally, remove plumbing to make it optional

All platforms use it at this point, including iOS which was the
original hold out for memory reasons. No more reason to make it
optional.

Updates #9332

Change-Id: I743fbc2f370921a852fbcebf4eb9821e2bdd3086
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -290,7 +290,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/paths                                          from tailscale.com/ipn/ipnlocal+
      💣 tailscale.com/portlist                                       from tailscale.com/ipn/ipnlocal
         tailscale.com/safesocket                                     from tailscale.com/client/tailscale+
-        tailscale.com/smallzstd                                      from tailscale.com/cmd/tailscaled+
+        tailscale.com/smallzstd                                      from tailscale.com/control/controlclient+
   LD 💣 tailscale.com/ssh/tailssh                                    from tailscale.com/cmd/tailscaled
         tailscale.com/syncs                                          from tailscale.com/net/netcheck+
         tailscale.com/tailcfg                                        from tailscale.com/client/tailscale/apitype+

cmd/tailscaled/tailscaled.go

@@ -48,7 +48,6 @@ import (
 	"tailscale.com/net/tstun"
 	"tailscale.com/paths"
 	"tailscale.com/safesocket"
-	"tailscale.com/smallzstd"
 	"tailscale.com/syncs"
 	"tailscale.com/tsd"
 	"tailscale.com/tsweb/varz"
@@ -551,9 +550,6 @@ func getLocalBackend(ctx context.Context, logf logger.Logf, logID logid.PublicID
 	if root := lb.TailscaleVarRoot(); root != "" {
 		dnsfallback.SetCachePath(filepath.Join(root, "derpmap.cached.json"), logf)
 	}
-	lb.SetDecompressor(func() (controlclient.Decompressor, error) {
-		return smallzstd.NewDecoder(nil)
-	})
 	configureTaildrop(logf, lb)
 	if err := ns.Start(lb); err != nil {
 		log.Fatalf("failed to start netstack: %v", err)