cpu/stm32: use common pm_off() function

The code is identical to the one found in sys/pm_layered/pm.c

cpu/stm32/include/periph_cpu.h

@@ -98,11 +98,6 @@ extern "C" {
 #define CPUID_ADDR          (UID_BASE)
 #endif
 
-/**
- * @brief   We provide our own pm_off() function for all STM32-based CPUs
- */
-#define PROVIDES_PM_LAYERED_OFF
-
 /**
  * @brief   All STM timers have 4 capture-compare channels
  */

cpu/stm32/periph/pm.c

@@ -151,9 +151,3 @@ void pm_set(unsigned mode)
 #endif
     }
 }
-
-void pm_off(void)
-{
-    irq_disable();
-    pm_set(0);
-}