staging: r8188eu: remove unused struct declarations

Some structures are only declared but have not been used anywhere
in the code. Remove such unused structs. Issue identified as part
of coccicheck report driven code investigation.

Suggested-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Deepak R Varma <drv@mailo.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/Y2LYvhr74ng+xFbz@qemulion


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