[nrf52480] Don't call PlatformDeinit() before NVIC_SystemReset() (#2385)

Calling `PlatformDeinit()` before calling `NVIC_SystemReset()` reduces
the reliability of `otPlatReset()` to always work properly, and doing so
seems superfluous if you are just going to reset the chip anyway.

This fixes issue #2383.

examples/platforms/nrf52840/misc.c

@@ -71,8 +71,6 @@ void otPlatReset(otInstance *aInstance)
 {
     (void)aInstance;
 
-    PlatformDeinit();
-
     NVIC_SystemReset();
 }