Remove rng seed call in platform_init().

It is not needed, as the random number generator is already seeded
in uBit.init().
This also works around this issue until it is resolved in DAL &
CODAL:
#5409

Apart from that it is a costly call, specially in V1 where it
takes 50 microsec or more, and in theory can take up to 480 us.

libs/core/codal.cpp

@@ -50,7 +50,6 @@ MicroBitEvent lastEvent;
 bool serialLoggingDisabled;
 
 void platform_init() {
-    microbit_seed_random();
     int seed = microbit_random(0x7fffffff);
     DMESG("random seed: %d", seed);
     seedRandom(seed);