clk: renesas: rcar-gen4: Remove unused fixed PLL clock types

All users of the fixed default PLL2/3/4/6 clock types have been
converted to fixed or variable fractional PLL clock types.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/c0229eb3518444f61173c6fb83bdcedb058dd079.1721648548.git.geert+renesas@glider.be