xProfile: change the field visibility JS to show/hide the controls immediately, rather than with a slide/fade transition.

This helps improve the perceived performance of the visibility
controls. The code quality of these parts of the JS has also been
improved.

Fixes #5439

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8021 cdf35c40-ae34-48e0-9cc9-0c9da1808c22