ARM: pxa: remove broken balloon3_gpio_vbus reference

balloon3_udc_init() tries to register a balloon3_gpio_vbus
device, but this has never been defined in the mainline
kernel. To avoid the obvious build failure when this function
is enabled, remove the bogus reference here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Daniel Mack <zonque@gmail.com>