Staging: rtl8188eu: Remove unused struct ndis_802_11_key

This patch removes struct ndis_802_11_key as it is defined
in a header file but never used.

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>