@sharonyue to remove the panel you have to edit the template.
https://github.com/NodeBB/nodebb-theme-persona/tree/master/templates
I do not know exactly where it is but it should something with topic template.
Maybe here:
https://github.com/NodeBB/nodebb-theme-persona/blob/master/templates/topic.tpl
<div class="topic-header">
<h1 component="post/header" class="" itemprop="name">
<span class="topic-title" component="topic/title">
<span component="topic/labels">
<i component="topic/pinned" class="fa fa-thumb-tack <!-- IF !pinned -->hidden<!-- ENDIF !pinned -->" title="{{{ if !pinExpiry }}}[[topic:pinned]]{{{ else }}}[[topic:pinned-with-expiry, {pinExpiryISO}]]{{{ end }}}"></i>
<i component="topic/locked" class="fa fa-lock <!-- IF !locked -->hidden<!-- ENDIF !locked -->" title="[[topic:locked]]"></i>
<i class="fa fa-arrow-circle-right <!-- IF !oldCid -->hidden<!-- ENDIF !oldCid -->" title="{{{ if privileges.isAdminOrMod }}}[[topic:moved-from, {oldCategory.name}]]{{{ else }}}[[topic:moved]]{{{ end }}}"></i>
{{{each icons}}}{@value}{{{end}}}
</span>
{title}
</span>
</h1>
<div class="topic-info clearfix">
<div class="category-item inline-block">
{{{ if category.icon }}}
<div role="presentation" class="icon pull-left" style="{{{ if category.bgColor }}}background-color: {category.bgColor};{{{end}}}; {{{ if category.color}}}color: {category.color};{{{end}}}">
<i class="fa fa-fw {category.icon}"></i>
</div>
{{{ end }}}
<a href="{config.relative_path}/category/{category.slug}">{category.name}</a>
</div>
<div class="tags tag-list inline-block hidden-xs">
<!-- IMPORT partials/topic/tags.tpl -->
</div>
<div class="inline-block hidden-xs">
<!-- IMPORT partials/topic/stats.tpl -->
</div>
{{{ if !feeds:disableRSS }}}
{{{ if rssFeedUrl }}}<a class="hidden-xs" target="_blank" href="{rssFeedUrl}"><i class="fa fa-rss-square"></i></a>{{{ end }}}
{{{ end }}}
<!-- IMPORT partials/topic/browsing-users.tpl -->
<!-- IMPORT partials/post_bar.tpl -->
</div>
</div>
