Remove 'asCompletableFuture' method from Styx Eventual.

[mikkokar]

Removes asCompletableFuture from Eventual. It feels quite pointless now as the Eventual implements a Reactive Streams Publisher interface.

In unit tests it is often useful to block and wait for response. Use Mono from Reactor-core instead:

HttpRequest full = Mono.from(streamingRequest.aggregate(ONE_MEGABYTE)).block();

Use Mono to convert to CompletableFuture:

Mono.from(response1.aggregate(1000)).toFuture();