Remove callback based LambdaHandler

[fabianfett]

We should remove the Callback LambdaHandler.

[saltzmanjoelh]

Hi, sorry to comment on an already closed issue. I wasn't sure if I should open a new issue or continue with this one?

I noticed that the readme has the closure based examples. Do you want some help updating them? If so, would you like them to simply use the async handle(event:context) form like in LambdaTestingTests.testCodableClosure?

Thanks!