When activity comments are deleted via AJAX, remove them from the DOM entirely

Previously, deleted comments were merely hidden using jQuery.fadeOut(). As a
result, when the show/hide comment button was toggled - applying $.show() to
all comment items - the comments would appear again, even though they had been
deleted from the server. Removing them from the DOM ensures that this won't
happen.

See #4844

Props merty

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7000 cdf35c40-ae34-48e0-9cc9-0c9da1808c22