mcs: remove getKernelWcetTicks from refill_ready

Previously, a refill would be ready if its head time
was at most getKernelWcetTicks after the current time.
This would mean that refill_unblock_check could
bring a refill's time forward, which violates an
invariant (namely that the time of the last refill
is at most the period from the time of the head refill).

Moreover, since the time to exit the kernel is always
less than the WCET, this might result in us running
a thread whose refill time is in the future, which
seems to violate the timing model.

Signed-off-by: Michael McInerney <michael.mcinerney@proofcraft.systems>