hrt: remove hrt_elapsed_time_atomic

There's no protection on posix

platforms/posix/src/px4/common/drv_hrt.cpp

@@ -157,20 +157,6 @@ hrt_abstime ts_to_abstime(const struct timespec *ts)
 	return result;
 }
 
-/*
- * Compute the delta between a timestamp taken in the past
- * and now.
- *
- * This function is safe to use even if the timestamp is updated
- * by an interrupt during execution.
- */
-hrt_abstime hrt_elapsed_time_atomic(const volatile hrt_abstime *then)
-{
-	// This is not atomic as the value on the application layer of POSIX is limited.
-	hrt_abstime delta = hrt_absolute_time() - *then;
-	return delta;
-}
-
 /*
  * Store the absolute time in an interrupt-safe fashion.
  *