regulator: ab8500: Remove ab8500_regulator_set_voltage_time_sel

All users of ab8500_regulator_volt_mode_ops and ab8500_regulator_volt_ops
do not set info->desc.enable_time, thus set_voltage_time_sel() always returns 0.
Remove it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>