Remove unused suspend_idle (#16063)

platforms/suspend.h

@@ -3,7 +3,6 @@
 #include <stdint.h>
 #include <stdbool.h>
 
-void suspend_idle(uint8_t timeout);
 void suspend_power_down(void);
 bool suspend_wakeup_condition(void);
 void suspend_wakeup_init(void);