ARM: pxa25x_udc: stop handling gpio-vbus internally

Stop handling gpio-vbus internally. All boards that depended on this
functionality have been converted to use gpio-vbus tranceiver. All
new boards can use it right from the start. Drop unused code.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>