Optmize field-related code for Drupal 7.22

http://drupal.org/node/1915646 seems to indicate that we could optimize the way that iCal presents Date and Location field choices.

However, if we do such an optmization, it would not be compatible with versions of Drupal before 7.22, unless the the changes are trivial enough to wrap in an if block that checks for the existence of the field_info_field_map() function.

I've set the priority of this task to minor, because I'm not even sure it'd be worthwhile to do. I think it would only affect the performance of the Views admin pages where Date iCal displays are used.