Consider dropping overloaded build method to receive custom engine

[jroper]

From @venkats:

In https://github.com/eclipse/microprofile-reactive/blob/master/streams/spec/src/main/asciidoc/architecture.asciidoc, under the Rationale section, in the paragraph that starts with "There are a number of different approaches..." the text reads: "...however users can select to use a custom implementation if they need." While this makes sense for the JDK implementation, does this make sense for MicroProfile implementation where 3-rd party dependency is not desired?

[jroper]

This is a good point, the API is already pluggable using the service loader mechanism, it doesn't necessarily need to be doubly pluggable. For the purposes of MicroProfile, I don't think we need the overload.

[Emily-Jiang]

This is only used by tck.