cpu/efm32/periph/timer: remove timer_set duplicate

timer_set is already provided by 'periph_common' so does not need to be
provided by cpu/periph.

cpu/efm32/periph/timer.c

@@ -98,11 +98,6 @@ int timer_init(tim_t dev, unsigned long freq, timer_cb_t callback, void *arg)
     return 0;
 }
 
-int timer_set(tim_t dev, int channel, unsigned int timeout)
-{
-    return timer_set_absolute(dev, channel, timer_read(dev) + timeout);
-}
-
 int timer_set_absolute(tim_t dev, int channel, unsigned int value)
 {
     TIMER_TypeDef *tim;