clk: renesas: cpg-mssr: Remove obsolete nullify check

All core clock nullify users and helpers were removed in commit
b1dec4e7 ("clk: renesas: rcar-gen3: Disable R-Car H3 ES1.*"),
but the CPG/MSSR driver still checks for nullified core clocks.
Remove the obsolete check.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/59ef3eccde0b0b63626480f27e77d5c68948ca98.1740126560.git.geert+renesas@glider.be