mfd: max77686: Remove unused struct max77686_opmode_data

The defined struct max77686_opmode_data isn't used neither by
the max77686 mfd driver nor the drivers for its sub-devices.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>