storage/index: stop tracking index objects in IndexTree
storage/index: stop tracking index objects in IndexTree

The previous changes to deleting empty indexes makes this unnecessary.

marineam/mantle,marineam/mantle,mjg59/mantle,quantum/coreos-mantle,dm0-/mantle,glevand/coreos--mantle,quantum/coreos-mantle,dm0-/mantle,glevand/coreos--mantle,mjg59/mantle