clk: renesas: rcar-gen3-cpg: Remove obsolete rcar_gen3_read_mode_pins()

All R-Car Gen3 clock drivers now obtain the values of the mode pins from
the R-Car RST driver.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>