upnp_context: unregister all FAILED mappings

This patch fixes a bug where FAILED mappings with the "auto-update"
option enabled are left in the local list, even after a new mapping has
been requested. This can cause the list to grow substantially over time
and was partly responsible for the bug described in the following issue:
jami-client-qt#1898

GitLab: #45
Change-Id: I009ff791ae90d10f64d46dede3b2cbfc9c1b7160