remove unused methods in driver.Scheduler

cinder.scheduler.host_manager.HostManager doesn't have get_host_list()
and get_service_capabilities(). But we don't catch the AttributeError
by them. Therefore, get_host_list() and get_service_capabilities() in
cinder.scheduler.driver.Scheduler have not been used.

Also, get_host_list() and get_service_capabilities() in
cinder.scheduler.manager.SchedulerManager have not been used.

Change-Id: I402cf068612866649cc8645391eff38fca56f634
Closes-Bug: #1236723
Merge "remove unused methods in driver.Scheduler"