staging: r8188eu: remove core/rtw_io.c

staging: r8188eu: remove core/rtw_io.c

There are only unused functions and macros and one function which can be
open-coded.

So, removed core/rtw_io.c at all, removed core/rtw_io.c from Makefile
and open-coded rtw_init_io_priv

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Co-developed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Link: https://lore.kernel.org/r/20210917071837.10926-14-fmdefrancesco@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>