drivers/pwm: remove PWM_NUMOF guard

as we do not build the periph drivers selectable currently,
the guard is triggered even for boards that do not have any
PWM defined and don't want to use it...

drivers/include/periph/pwm.h

@@ -33,13 +33,6 @@
 extern "C" {
 #endif
 
-/**
- * @brief   Make sure the number of available PWM devices is defined
- */
-#ifndef PWM_NUMOF
-#error "PWM_NUMOF undefined for the target platform"
-#endif
-
 /**
  * @brief   Default PWM access macro
  * @{