sched: fix locking of remove_vcpu() in credit1

sched: fix locking of remove_vcpu() in credit1

In fact, csched_vcpu_remove() (i.e., the credit1
implementation of remove_vcpu()) manipulates runqueues,
so holding the runqueue lock is necessary.

However, the vCPU just can't be on the runqueue, when
the function is called. We can therefore ASSERT() that,
and avoid doing any runqueue manipulations (rather than
adding the runqueue locking around it).

Also, while there, *_lock_irq() (for the private lock) is
enough, there is no need to *_lock_irqsave().

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>