Performed cleanup of Timer's free functions (#776)

* GetClock() is now officially deprecated since its comment already informally
  did so and the function just forwards to Timer::GetFPGATimestamp().
* The declaration of GetPPCTimestamp() is missing a definition and has been
  removed.
* std::this_thread::sleep_for() returns immediately when given a negative
  duration, so the check for that was removed from Wait().

wpilibc/src/main/native/cpp/Timer.cpp

@@ -28,7 +28,6 @@ namespace frc {
  * @param seconds Length of time to pause, in seconds.
  */
 void Wait(double seconds) {
-  if (seconds < 0.0) return;
   std::this_thread::sleep_for(std::chrono::duration<double>(seconds));
 }