Deprecate conductor instance_type_get() and remove from VirtAPI

The flavor_get() in VirtAPI and instance_type_get() in conductor are
no longer needed by the virt drivers. This patch deprecates the
function in conductor's API and removes it everywhere else.

Related to blueprint compute-manager-objects

Change-Id: I782ee449df98a198f625b946c650ebff56711b8e
Merge "Deprecate conductor instance_type_get() and remove from VirtAPI"