DPP: Delete PKEX code and identifier on success completion of PKEX

DPP: Delete PKEX code and identifier on success completion of PKEX

We are not supposed to reuse these without being explicitly requested to
perform PKEX again. There is not a strong use case for being able to
provision an Enrollee multiple times with PKEX, so this should have no
issues on the Enrollee. For a Configurator, there might be some use
cases that would benefit from being able to use the same code with
multiple Enrollee devices, e.g., for guess access with a laptop and a
smart phone. That case will now require a new DPP_PKEX_ADD command on
the Configurator after each completion of the provisioning exchange.

Signed-off-by: Jouni Malinen <quic_jouni@quicinc.com>