xfce4-power-manager: remove udisks rdependency

udisks spin down feature support was removed in:

commit dc6ef998a2f19b6c2fcbfb48164db04be44c885d
Author: Harald Judt <h.judt@gmx.at>
Date:   Sun Jul 13 21:34:22 2014 +0200

    Remove disk spin down feature (3/3)

    Finally, let's remove the low-level code and remaining definitions.

commit c9d0626110e4b6e906610e9a2316cdac7a11d58e
Author: Harald Judt <h.judt@gmx.at>
Date:   Sun Jul 13 21:26:21 2014 +0200

    Remove disk spin down feature (2/3)

    Remove all related GUI widgets from the settings dialog.

commit 3ad75fad293fb66e6316aa5820ee064f264eafa4
Author: Harald Judt <h.judt@gmx.at>
Date:   Sun Jul 13 21:22:05 2014 +0200

    Remove disk spin down feature (1/3)

    The spin down feature does not work and would need a rewrite
    to work with udisks-2. What's more, it is probably functionality
    best handled by other tools or scripts, so let's remove it from
    the power manager.

    As a first step, this commit removes all interfacing code between
    GUI and low-level udisks functions.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>