pinctrl: palmas: remove non-require function

Palmas pinmux and pin configuration support the single pin level
configuration in place of pin group.

Hence it is only require to pin_config_{set|get} and do not require
pin_config_group_{set|get}. As core framework already check for
require APIs availability, it is not require to implement as dummy
for non-require ops and so removing it.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>