core: Removed useless code from test

In UpdateVmDiskCommandTest, there were recurrent mockings to
vmDeviceDao.get, which were never called in the command, therefore the
mock is useless. Now removed, along with vmDevices that are no longer
used.

Change-Id: If1580f66cc5a41583abbc936308946ea3b70939f
Signed-off-by: Vered Volansky <vvolansk@redhat.com>