ios: stop linking in the websocket code?

The iOS client doesn't use websockets. In fact, only the js/wasm browser client does, for the Tailscale SSH console.

But we link it in on iOS:

```
bradfitz@book1pro tailscale.com % GOOS=ios GOARCH=arm64 ~/go/bin/deppath ./tstest/iosdeps /github.com/coder/websocket/internal
["./tstest/iosdeps" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "github.com/coder/websocket" "github.com/coder/websocket/internal/errd"]
["./tstest/iosdeps" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "github.com/coder/websocket" "github.com/coder/websocket/internal/util"]
["./tstest/iosdeps" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "github.com/coder/websocket" "github.com/coder/websocket/internal/xsync"]
["./tstest/iosdeps" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "tailscale.com/net/wsconn" "github.com/coder/websocket" "github.com/coder/websocket/internal/errd"]
["./tstest/iosdeps" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "tailscale.com/net/wsconn" "github.com/coder/websocket" "github.com/coder/websocket/internal/util"]
["./tstest/iosdeps" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "tailscale.com/net/wsconn" "github.com/coder/websocket" "github.com/coder/websocket/internal/xsync"]
["./tstest/iosdeps" "tailscale.com/ipn/localapi" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "github.com/coder/websocket" "github.com/coder/websocket/internal/errd"]
["./tstest/iosdeps" "tailscale.com/ipn/localapi" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "github.com/coder/websocket" "github.com/coder/websocket/internal/util"]
["./tstest/iosdeps" "tailscale.com/ipn/localapi" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "github.com/coder/websocket" "github.com/coder/websocket/internal/xsync"]
["./tstest/iosdeps" "tailscale.com/ipn/localapi" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "tailscale.com/net/wsconn" "github.com/coder/websocket" "github.com/coder/websocket/internal/errd"]
["./tstest/iosdeps" "tailscale.com/ipn/localapi" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "tailscale.com/net/wsconn" "github.com/coder/websocket" "github.com/coder/websocket/internal/util"]
["./tstest/iosdeps" "tailscale.com/ipn/localapi" "tailscale.com/ipn/ipnlocal" "tailscale.com/control/controlclient" "tailscale.com/control/controlhttp" "tailscale.com/net/wsconn" "github.com/coder/websocket" "github.com/coder/websocket/internal/xsync"]
```

We should not.

Related to https://github.com/tailscale/corp/issues/20099

/cc @raggi @oxtoacart @agottardo @barnstar