Removed duplicate pulseIn declaration.

Arduino.h contains multiple declarations of `pulseIn`, however one is declared `extern "C"` and the other without it.

As the C/C++ link protection is not an exclusive block, only the one declaration is needed.

```C++
#ifdef __cplusplus
extern "C"{
#endif

unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout = 1000000UL);

#ifdef __cplusplus
} // extern "C"
#endif
```