light-client: consolidate and delete old implementations

[xla]

With the advent of the light-client crate, which was meant to superseed prior implementations we now have a large corpus of code that is outdated and should be sun-setted. Keep maintaining this parts like lite and lite_impl is gonna add significant overhead that is unnecessary at this stage of the project.

These modules should be carefully evaluated to extract knowledge, implementation details which are worth preserving w.r.t. #416.

[xla]

@yihuang @joe-bowman this could be relevant to you.###

[liamsi]

I'm strongly in favour of this. While @ebuchman rightfully mentioned that other projects are already using the lite module (cc @yihuang @joe-bowman), I think it will start being confusing with the next release (there is lite, lite_impl, light-client, light-node etc).

Also, I already added a ⚠️ ️deprecation warning ⚠️ in one the last releases:https://github.com/informalsystems/tendermint-rs/blob/master/CHANGES.md#0140-2020-06-19

Note, that there already is a related issue #342 (we might want to deduplicate the deduplication issues too).

These modules should be carefully evaluated to extract knowledge, implementation details which are worth preserving w.r.t. #416.

I agree that the current lite module might help inform the steps towards #416 but I think it could be OK to have this captured here and in version control instead of keeping the code around.