maint: remove redundant disk_cachemode validation entry

* nova/virt/libvirt/driver.py (LibvirtDriver.__init__):
Remove a redundant duplicate entry, which would just
slow checks down slightly.

Change-Id: I7b5f7bf2cb00a45f88dbd3e3b6b4e339af49c8d8
Merge "maint: remove redundant disk_cachemode validation entry"