serial: sh-sci: Remove obsolete comment about overrun detection

The code it refers to was removed in commit b545e4f4 ("serial:
sh-sci: Compute overrun_bit without using baud rate algo").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>