pinctrl: mediatek: Drop mtk_pinconf_bias_set_pd()

This function is unused and causing compile errors, delete it.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/linux-next/20250106164630.4447cd0d@canb.auug.org.au/


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>