Fixed field token implementation.

This is very likely not properly working yet, but if fixes the fatal errors after https://drupal.org/node/2167167.

Removed a useless helper function because fields are entity type specific now so all that logic there was bogus and was trying to expose fields for bundles and not entity types.

With this, the field tokens now actually show up in the browser, I haven't tested if they work. Probably not :)