Remove unneeded #ifdef in SoftwareSerial

The debugPulse function definition already checks for _DEBUG, resulting
in an empty function definiton and the function call being optimized
away.

hardware/arduino/avr/libraries/SoftwareSerial/SoftwareSerial.cpp

@@ -175,9 +175,7 @@ void SoftwareSerial::recv()
     } 
     else 
     {
-#if _DEBUG // for scope: pulse pin as overflow indictator
       DebugPulse(_DEBUG_PIN1, 1);
-#endif
       _buffer_overflow = true;
     }