clk: renesas: rcar-gen4: Remove unused default PLL2/3/4/6 configs

The default PLL2/3/4/6 multiplier and divider configurations are no
longer used after the conversion to fixed or variable fractional PLL
clock types.

Note that the default configurations are still documented in the
comments above the individual rcar_gen4_cpg_pll_config instances.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/d13526a86066992d6afdf9bee7c1a18da72f914f.1721648548.git.geert+renesas@glider.be