remove awa-lwt: it has not been used much, and we want to focus on awa-mirage

[dinosaure]

/cc @palainp Are you fine with that?

[hannesm]

also cc @haesbaert who authored this code

[palainp]

I probably am, I've pretty much only used awa-mirage so far :) The original awa-lwt code was heavily used to write/update the awa-mirage part, so there are links between the two.

[hannesm]

I talked with christiano, he's fine this to be deleted.