staging:rtl8192u: remove unused structure tx_desc_819x_usb_aggr_subframe

Structure tx_desc_819x_usb_aggr_subframe is defined in a local header file but
is not used outside of the header file. Removed from the code as a result.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>