Staging: rtl819u: ieee80211: Remove function ieee80211_wep_null

Remove function ieee80211_wep_null from the file as is it blank and
remove its declaration from the header file.
Also remove its function call as it is of no use calling this function.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>