This reverts commit f028ca4edd.
In the Train PTG[1] we agreed to defer this feature until we had some
kind of traceability or discoverability for APIs and that this wasn't
feasible or useful until then.
This change was merged to master but never released, so I submit that
it is safe to revert.
[1] https://etherpad.openstack.org/p/keystone-train-ptg-application-credentials
Change-Id: I8fc5fcb2b35431882f0d64866765d6b0cd31356f