AS3 apollo-server-lambda: drop callback support

[glasser]

As of recently, the handler in apollo-server-lambda can be called either with a callback or as an async function. We believe that all currently running Lambda Node runtimes support the async version. Now, some folks may be wrapping our handler with their own handler and calling it callback-style, but AS3 is a fine time to drop the complexity (including weirder types, see #5072) and only export an async handler.

[glasser]

Duplicate of #5018