power: pmic_max77686: remove the old pmic_max77686 file

max77686 pmic is supporting with max77686.c under pmic/ and regulator/
direnctroy. Remove pmic_max77686.c what didn't use anywhere.
Instead, enable CONFIG_DM_REGULATOR_MAX77686 and
CONFIG_DM_PMIC_MAX77686.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>