regulator: ab8500-ext: Remove get_voltage to avoid duplicate implementation

The implementation of ab8500_ext_fixed_get_voltage is identical to
ab8500_ext_list_voltage. We can avoid the duplicate implementation by just
remove get_voltage. For fixed regulator, regulator core will call
list_voltage(rdev, 0) to get voltage if both get_voltage get_voltage_sel are
not implemented.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>