core: eliminate unnecessary tcb_t struct fields

PR by @kaspar030 and me: Some of tcb_t's members are not required.
You don't need to know it. [sched.c:72](https://github.com/RIOT-OS/RIOT/blob/b60483277722d657c6058762d969184340bfd344/core/sched.c#L72) only tests if someone killed the canary.