ipn/ipnserver: revert decoder memory limit.

The zstd library treats that limit as a hard cap on decompressed
size, in the mode we're using it, rather than a window size.

Signed-off-by: David Anderson <danderson@tailscale.com>

ipn/ipnserver/server.go

@@ -116,7 +116,6 @@ func Run(rctx context.Context, logf logger.Logf, logid string, opts Options, e w
 		return zstd.NewReader(nil,
 			zstd.WithDecoderLowmem(true),
 			zstd.WithDecoderConcurrency(1),
-			zstd.WithDecoderMaxMemory(65536),
 		)
 	})