ipw2x00: remove unused ->ibss_dfs pointer

The ->ibss_dfs pointer is always allocated with a user controlled
length.  This caused a static checker warning because what if the length
was zero?  In that case, any dereference of ->ibss_dfs would lead to an
Oops.

It turns out that this isn't a problem because the ->ibss_dfs pointer is
never used.  This patch deletes it along with all the related code.  In
particular the entire libipw_network_reset() function can be removed.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>