dt-bindings: usb: Remove vconn_uses_external_source from qpnp-pdphy

The flag was introduced to support board designs where the PMIC's
VCONN input was connected directly to VBUS, and to prevent damage
to the Type-C connector if VBUS was greater than 5V. This is no
longer recommended to do in current and future board designs, so
remove this property as VCONN will always be independent from VBUS.

Change-Id: I0210ca0a81fd174d41764a6549dd0ef44695e058
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>