Removes custom CONFIG_ATHENA compiler directive (#796)

In roborio GCC 5.5, __FRC_ROBORIO__ has been added directly to the
compiler. So we can instead use that to detect if we are build for the
roborio, and in a much more reliable way.

myRobot/src/main/native/cpp/MyRobot.cpp

@@ -7,7 +7,7 @@
 
 #include "IterativeRobot.h"
 
-class MyRobot : public IterativeRobot {
+class MyRobot : public frc::IterativeRobot {
   /**
    * This function is run when the robot is first started up and should be
    * used for any initialization code.