staging: r8188eu: remove unused timer functions

rtw_get_passing_time_ms and rtw_systime_to_ms are not used any more.
Remove them.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220413193654.258507-9-martin@kaiser.cx


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