Do not zero out sched_threads needlessly

The function sched_init() zeroes out sched_threads needlessly. All
static variables can be assumed to be initialized with zero, anyways.
The C standard mandates it, and all at other places in the code it is
assumed.

Currently, if your CPU or board spawns worker threads before calling
kernel_init(), they get deleted.
[Bleeding edges] Dump UHCI worker thread implementation