core: remove unneeded static method declarations

In AttachUserToVmFromPoolAndRunCommand class.
and a small clean up to replace
DbFacade.getInstance().getVmPoolDao() with getVmPoolDao().

Change-Id: I3c9b3d50e106583970995a157913a1b6960d1d42
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>