Fix wakeup_remove

[Calvin-LL]

Hi, I found this bug while working on the class project in @achreto's class at UBC.

Consider this code snippet:

struct dcb *dcb = ...;
wakeup_set(dcb, 3);
wakeup_remove(dcb);
wakeup_set(dcb, 4);

The second call to wakeup_set would cause this assertion to fail

barrelfish/kernel/wakeup.c

Line 42 in 06a9f54

assert(kcb_current->wakeup_queue_head == dcb);

The function uses dcb->wakeup_time to tell if a dcb is on the wakeup queue but doesn't set it back to 0 when removing a dcb. This PR fixes that.