janitorial: drop stray refs to storage.misc.execCmd

storage.misc.execCmd was moved to vdsm.utils quite long ago. Drop
references from outside the storage subdir to the old location.
Further references (from within storage) should be dropped in a future
patch.

Change-Id: Iea6fd4f401adb6273e4106525893cc4ccd6c73cf
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22219
Reviewed-by: Francesco Romani <fromani@redhat.com>
Tested-by: Francesco Romani <fromani@redhat.com>