xProfile: fix regression in wp-admin field types screen where duplicate field templates were being shown for pre-2.0 custom field types.

A switch statement in the original `xprofile_admin_field()` was mis-read and an action was being triggered when it shouldn't have, causing the field templates to double up.
Fixes backwards compatibility with existing plugins that add custom field types.

Fixes #5511

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8232 cdf35c40-ae34-48e0-9cc9-0c9da1808c22