usb:hsotg:samsung: Remove Disconnect Interrupt handler

The USB Disconnect Interrupt handler, according to specification,
is only working at HOST mode.

Samsung SoCs (e.g. Exynos4) are working at device mode, so
this interrupt is never caught.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>