Remove unused PolicyFileNotFound exception
Remove unused PolicyFileNotFound exception

I could not find any use of this exception in Neutron,
nor in any other project. It has a bad message: the policy
file name is hard coded. The policy file name is a config
option so it could be anything.

It should have been deleted with https://review.openstack.org/123673

Discovered while picking exceptions to move to neutron-lib.

Change-Id: I4732d84186520be3286417579ee3e2528fcf53a3

noironetworks/neutron,sebrandon1/neutron,eayunstack/neutron,cloudbase/neutron,igor-toga/local-snat,noironetworks/neutron,mahak/neutron,cloudbase/neutron,mahak/neutron,openstack/neutron,bigswitch/neutron,klmitch/neutron,MaximNevrov/neutron,sebrandon1/neutron,klmitch/neutron,huntxu/neutron,openstack/neutron,MaximNevrov/neutron,igor-toga/local-snat,eayunstack/neutron,mahak/neutron,openstack/neutron,huntxu/neutron,bigswitch/neutron