Sorry – the file is form-topic.php
and you have to remove the same code from form-reply.php
I just had the same problem but figured it out.
Probably not the cleanest solution but it works
go to the bbp-themes/bbp-twentyten/bbpress/ folder
open forum-topic.php
search for “form-allowed-tags”
select and delete
<?php if ( !current_user_can( 'unfiltered_html' ) ) : ?>
<p class="form-allowed-tags">
<label><?php _e( 'You may use these <abbr title="HyperText Markup Language">HTML</abbr> tags and attributes:','bbpress' ); ?></label>
<code><?php bbp_allowed_tags(); ?></code>
</p>
<?php endif; ?>
