remove redundant __init__() overwriting when getting ExtensionResources

As described in #1266344, some of child class in
<nova>/nova/api/openstack/compute/contrib/ has overwrited __init__()
without specified needs. Removing that could help keeps codes clean and
consistent (nearly all others don't overwrite __init__()).

Change-Id: I87dfd9c1622bc8250f620ef591391a0e98573f78
Closes-Bug: #1266344
Merge "remove redundant __init__() overwriting when getting ExtensionResources"