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/Notifier.cpp

@@ -103,7 +103,7 @@ void Notifier::StartSingle(double delay) {
   std::lock_guard<wpi::mutex> lock(m_processMutex);
   m_periodic = false;
   m_period = delay;
-  m_expirationTime = GetClock() + m_period;
+  m_expirationTime = Timer::GetFPGATimestamp() + m_period;
   UpdateAlarm();
 }
 
@@ -121,7 +121,7 @@ void Notifier::StartPeriodic(double period) {
   std::lock_guard<wpi::mutex> lock(m_processMutex);
   m_periodic = true;
   m_period = period;
-  m_expirationTime = GetClock() + m_period;
+  m_expirationTime = Timer::GetFPGATimestamp() + m_period;
   UpdateAlarm();
 }
 

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));
 }