Kernel: Removed the priority boost code for starved threads.

After hwtesting and reverse engineering the kernel, it was found that the CTROS scheduler performs no priority boosting for threads like this, although some other forms of scheduling priority-starved threads might take place.

For example, it was found that hardware interrupts might cause low-priority threads to run if the CPU is preempted in the middle of an SVC handler that deschedules the current (high priority) thread before scheduling it again.