libertas: remove dump_survey implementation

libertas provides a dump_survey implementation based on reading of
a RSSI value. However, this RSSI value is calculated based on the
last received beacon from the associated AP - it is not a good
way of surveying a channel in general, and even causes an error
if the card is not associated to a network.

As this is not appropriate as a survey, remove it. This fixes an
issue where something in userspace is repeatedly calling site-survey
during boot, resulting in many repeated errors as the RSSI value cannot
be read before associating.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>