Remove unused permitted_instance_types

Commit 5ea7db9b removed the ArchFilter and moved its functionality
to the ImagePropertiesFilter, which uses 'supported_instances' from
the compute nodes to determine if a node satisfies the arch,
preferred hypervisor, and virtual machine mode requested by the
instance.  permitted_instance_types is no longer used and can be
removed.

Change-Id: Idf9d72746d7c3dcd2a7385913823df122eaa7cd6

nova/tests/test_libvirt.py

@@ -2279,7 +2279,6 @@ class LibvirtConnTestCase(test.TestCase):
 
         want = {"vendor": "AMD",
                 "features": ["extapic", "3dnow"],
-                "permitted_instance_types": ["x86_64", "i686"],
                 "model": "Opteron_G4",
                 "arch": "x86_64",
                 "topology": {"cores": 2, "threads": 1, "sockets": 4}}