Use system mqtt library

[Gadgetoid]

Is there a reason (other than the timeout change) why we're using https://github.com/pimoroni/enviro/blob/main/enviro/mqttsimple.py in lieu of the built-in mqtt library?

It might be possible to upstream this timeout change (it looks non-breaking) and rely on the micropython-lib version of the library that's baked-in using Pico W Enviro's manifest.py:

https://github.com/pimoroni/pimoroni-pico/blob/c00fee673ec5b6e44e69c55c8c550532f59adf6e/micropython/_board/picow_enviro/PICO_W_ENVIRO/manifest.py#L8

This would save us some user filesystem space.