usb: dwc3-msm: Remove ext_vbus power_supply

The ext-vbus power_supply was intended to relay clients of
usb power_supply changes. However there are no current users
and it is therefore dead code. Remove it.

Change-Id: Icf6493db82d8b44d9262d8dac153b0c6f3df8545
Signed-off-by: Jack Pham <jackp@codeaurora.org>