staging: r8188eu: remove readmem and writemem prototypes

Remove the prototypes for the unused _rtw_read_mem and _rtw_write_mem
functions.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20230111195640.306748-14-martin@kaiser.cx


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