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 #33 #7 #35