usb: pd: Remove vconn_is_external flag

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 flag and associated checks as we are safe to enable
VCONN independently of what VBUS is operating at.

Change-Id: I7697947e590f9e1eaf6958ebd157dbc41a8c5cdc
Signed-off-by: Jack Pham <jackp@codeaurora.org>