Don't return options list for term reference fields with free tagging

For taxonomy term reference fields with free tagging enabled, the options list can contain arbitrary numbers of tags an therefore be really huge. I ran into problems because of this in #1129262: PDOException when enabling a taxonomy term field with multiple values, and generally I can imagine few use cases where an options list for a free tagging field would be useful.
Therefore, there probably shouldn't be a options list returned for such fields.