Eliminate OVERRIDE_11 and FINAL_11 macros.

Remove the macro definitions from Simbody, and replace `OVERRIDE_11 -> override` and `FINAL_11 -> final` everywhere. We said Simbody 3.6 and above must be compiled with a C++11 compiler. This seals the deal.

**Reviewer question**: is OpenSim still depending on Simbody to define these macros? Should I leave the macros defined unconditionally to `override` and `final` in case anyone is still using them?