staging: r8188eu: delete unused header

The iee80211_ext.h header is unused and can be removed.

Signed-off-by: Zameer Manji <zmanji@gmail.com>
Link: https://lore.kernel.org/r/20211116193557.1188939-1-zmanji@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>