smdk5250: Remove 'board_usb_vbus_init()' function

Previously as a part of moving the VBUS gpio support to device tree
following patch removed this and added relevant support in driver:
4a271cb1 exynos: usb: Switch USB VBUS GPIOs to be device tree configured

Recent changes for common board file migration for exynos platform
added it again. So removing it now.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Cc: Julius Werner <jwerner@chromium.org>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>