[xfconf] Remove unused bbappend.

Resolved by this commit on meta-openembedded:
    commit 480d3338a7401f53e7b8a031294d37498a6a7be6
    Author: ulf@emagii.com <ulf@emagii.com>

See http://comments.gmane.org/gmane.linux.distributions.angstrom.devel/7364

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Set openssh as a PREFERRED_PROVIDER for sshd

Both dropbear and openssh are valid providers for sshd. As there is no
distro policy at the moment, we pick openssh in the local.conf.sample.

Signed-off-by: Ash Charles <ashcharles@gmail.com>