Remove ptp from persist config on boot

We got a report from a user in which ptp was set in the
persistent config, likely from a previous version.
This causes errors in the usb state and is not removed
by an ota. To fix, remove ptp in the same place that mtp
is removed from the persistent config.

Bug: 62202885
Test: Add ptp to persistent config, verify removed.
Change-Id: I5ebd93b9c8a49bcaca5a3362e49ed1e1acf50a9b