Remove unused instance variables from HostState

The request was to fix num_instances_by_project in HostState to
correctly report state even when last VM was deleted.

However in investigation I discovered this instance variable isn't
used in openstack code.  Following policy, we should be removing this
code.  Also removed vm_states, task_states and
num_instances_by_os_type.

Change-Id: I4ba30a9c0639803d19d5e8355f5bc8dd3f543671
Closes-Bug: 1201379
Merge "Remove unused instance variables from HostState"