core: remove unnecessary tests in sched.c

The scheduling gets activated by `kernel_init()` calling
`cpu_switch_context_exit()`. Before this `sched_run()` won't be called.
When it gets called, at least the main thread and the idle thread are
spawned. The idle thread won't die / get killed. So there always is at
least one thread in `runqueue_bitcache`.

Closes #19.