Dropping RTT::OS::RunnableInterface

I'm thinking about removing the rtt/os/RunnableInterface.hpp and only
keep rtt/RunnableInterface.hpp. No user should be using it, and it
isn't used by RTT. I could/will keep this change for the RTT 2.0
branch only, but it's just a wast of bytes.

Any complaints, let's hear 'em.

Peter