Change 'Delete Message' to 'Delete Conversation' on message delete title button.

This makes it clearer what the button actually does.

Props SGr33n, joshshashaty.
Fixes #5908.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9089 cdf35c40-ae34-48e0-9cc9-0c9da1808c22