This patch removes a handful of unused exceptions from
neutron.common.exceptions as follows:
The following exceptions already live in neutron-lib and are thus
duplicates in neutron:
PlacementEndpointNotFound
PlacementResourceProviderNotFound
PlacementInventoryNotFound
PlacementAggregateNotFound
PlacementInventoryUpdateConflict
NetworkVlanRangeError
PhysicalNetworkNameError
MacAddressGenerationFailure
The following exception is not used anywhere today:
TenantNetworksDisabled
NeutronLibImpact
Change-Id: Ie8227954681292ead1da0dedc950c54028ce84f1