regulator: ab8500: Remove USB regulator

The USB regulator is controlled by hardware. The software support
was only needed for early hardware (ED) which is no longer supported.

Signed-off-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>