Improve JS for removing items from message recipient autocomplete list

This changeset introduces a more reliable method for removing usernames from
the list of message recipients, which works properly with usernames that
contain hyphens.

Fixes #4779

Props mort3n

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6928 cdf35c40-ae34-48e0-9cc9-0c9da1808c22