Following b3c89e8c and ccb3d340 fix unused links to old external-auth
page and remove unused dangling anchor.
Change-Id: I2a067cd2e3ac3217eb4e2435539968fd6f2691cc
Some of the admin guide pages were prefixed with "identity-" because
they came from the centralized install guide or operators guide or
security guide or somewhere else. We don't need the prefix, everything
in keystone is identity. Remove the prefix from the affected pages so
that everything is consistent.
Change-Id: Icd172a39fe720472f1fb15395178f90282696ac9