net/wireless: ipw2x00: remove unused libipw_measurement_report struct

and all referenced structs and corresponding enums because the driver
does not use it.

Note: keep libipw_info_element struct since it is still in use.

Signed-off-by: Stanislav Yakovlev <stas.yakovlev@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>