Remove LWIP from pico-extras ?

[lurch]

Now that lwIP has been integrated into pico-sdk itself (as part of the Pico W functionality), perhaps it makes sense to remove it from pico-extras ? And for the lwIP-related issues and PRs in this repo to either be closed, or transferred to pico-sdk, as appropriate?
#19
#20
#27
raspberrypi/pico-sdk#1244
#7
#35

[lurch]

Fixed by 5e594d4

@kilograham You might want to go through and close all the other lwIP-related issues in my comment above?

[kilograham]

ah thanks, i had closed some of those but not all!