Old Yubikey-specific code and settings need to be removed

The old, experimental, Yubikey code and settings are still lurking in at least the schema.py and main.cfg files in both trunk and the sso-config branch. We can't remove the section independently as it's defined as 'fatal=True' so we'll need to correlate removal and and deployment. There may also be removable Yubi code in other areas and in the templates. This has all been superseded by the OATH 2-factor code so should be removed at some point.