clang-tidy: partially fix readability-redundant-declaration

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

@@ -72,10 +72,8 @@ static LockstepScheduler *lockstep_scheduler = new LockstepScheduler();
 #endif
 
 
-hrt_abstime hrt_absolute_time_offset();
 static void hrt_call_reschedule();
 static void hrt_call_invoke();
-__EXPORT hrt_abstime hrt_reset();
 
 hrt_abstime hrt_absolute_time_offset()
 {
@@ -463,11 +461,6 @@ void	hrt_call_at(struct hrt_call *entry, hrt_abstime calltime, hrt_callout callo
 	hrt_call_internal(entry, calltime, 0, callout, arg);
 }
 
-/*
- * Convert absolute time to a timespec.
- */
-void	abstime_to_ts(struct timespec *ts, hrt_abstime abstime);
-
 static void
 hrt_call_invoke()
 {