Remove obsolete RxJava code and keep RxJava2 only one

[Sergey Beryozkin]

Hi

cxf-rt-rs-extension-rx ships the code for both (old) RxJava and RxJava2 
code. It supports returning RxJava2 Flowable and Observable on the 
server and accepting it on the client, and the same for the (old) RxJava
Observable...
While even the (old) RxJava code is very new for CXF, the reality is 
that RxJava has been around for a while now and with RxJava2 embracing 
org.reactivestreams, it's hard to see CXF users preferring to start with 
the (old) RxJava.

The other minor problem is that cxf-rt-rs-extension-rx has optional 
RxJava and RxJava2 deps to be able to ship the relevant code in the same 
module and splitting it into 2 modules will be too much at this point.

I suggest that unless some users confirm (I CC to the users) that they 
need to use the (old) RxJava code, then we just remove it and make 
things much simpler...

Thanks, Sergey