wifi: libertas: stop selecting wext

Libertas no longer references the iw_handler infrastructure or wext_spy,
so neither of the 'select' statements are used any more.

Fixes: e86dc1ca

 ("Libertas: cfg80211 support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231108153409.1065286-1-arnd@kernel.org