libvirt: remove unused 'get_disks' method

The 'get_disks' method was a virt driver public API used by
the old compute monitor class. This class was deleted in

  commit 07646e85841a4f7c81e80254ac63715bece2aadd
  Author: Brian Waldon <brian.waldon@rackspace.com>
  Date:   Tue Aug 2 10:09:58 2011 -0400

      removing compute monitor

Leaving no in-tree user of the 'get_disks' method. This
method is not declared in the ComputeDriver base class
nor implemented by any virt driver besides libvirt, so
can be safely deleted.

Partial-bug: #1333219
Change-Id: I4d3f53499d088a82e471ab68feef1ae50d3f8bdc
Merge "libvirt: remove unused 'get_disks' method"