ARM: tegra: acer-a500: Remove bogus USB VBUS regulators

The configuration of USB VBUS regulators was borrowed from downstream
kernel, which is incorrect because the corresponding GPIOs are connected
to PROX_EN (A501 3G model) and LED_EN pins in accordance to the board
schematics. USB works fine with both GPIOs being disabled, so remove the
bogus USB VBUS regulators. The USB VBUS of USB3 is supplied from the fixed
5v system regulator and device-mode USB1 doesn't have VBUS switches.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>