libertas: remove mesh_autostart_enabled and sync_channel

mesh_autostart_enabled was nowhere set. Rumor is that this is used in the
OLPC tree, but they never did submit their code upstream.

After removing this code, it turned out that the sync_channel stuff is now
also unused, so get rid of that as well.

Signed-off-by: Holger Schurig <holgerschurig@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>