cpu/cortexm_common: remove special cortexm_sleep handle for stm32l152re

__set_PRIMASK(state) had been directly inlined to avoid a hardfault that
occured when branching after waking up from sleep with DBG_STANDBY,
DBG_STOP or DBG_SLEEP set in DBG_CR.

The hardfault occured when returning from the branch to irq_restore,
since the function is now inlined the branch does not happen either.

See RIOT-OS#14015 for more details.

cpu/cortexm_common/include/cpu.h

@@ -170,12 +170,7 @@ static inline void cortexm_sleep(int deep)
     unsigned state = irq_disable();
     __DSB();
     __WFI();
-#if defined(CPU_MODEL_STM32L152RE)
-    /* STM32L152RE crashes if branching to irq_restore(state). See #11830. */
-    __set_PRIMASK(state);
-#else
     irq_restore(state);
-#endif
 }
 
 /**