No reliable way at compile time to determine if you're using LwIP

[peterharperuk]

We mark all the LwIP libraries with NO_FLAG. CYW43 provides CYW43_LWIP=1 but you can use LwIP without CYW43. And this flag is defined by the pico_cyw43_arch* libraries which you don't have to use. Or you can use CYW43 with your own LwIP implementation.

I think we should remove NO_FLAG from pico_lwip as otherwise there's no way to conditionally compile code that may or may not use LwIP without using the CYW43 libraries.