Revert r9244: "Templates: add class to bp-legacy's single forum reply template (legacy bbPress) to enable @mentions."

Further consideration is needed before adding features to the legacy Group Forums support. See #5934


git-svn-id: https://buddypress.svn.wordpress.org/trunk@9245 cdf35c40-ae34-48e0-9cc9-0c9da1808c22