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/tsconnect/wasm/wasm_js.go

@@ -35,7 +35,6 @@ import (
 	"tailscale.com/net/netns"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/safesocket"
-	"tailscale.com/smallzstd"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tsd"
 	"tailscale.com/wgengine"
@@ -133,9 +132,6 @@ func newIPN(jsConfig js.Value) map[string]any {
 	if err := ns.Start(lb); err != nil {
 		log.Fatalf("failed to start netstack: %v", err)
 	}
-	lb.SetDecompressor(func() (controlclient.Decompressor, error) {
-		return smallzstd.NewDecoder(nil)
-	})
 	srv.SetLocalBackend(lb)
 
 	jsIPN := &jsIPN{