Remove obsolete class attribute from sidebar's splitter.

mailnews/base/resources/content/messenger.xul

@@ -310,7 +310,7 @@ Rights Reserved.
 	<box align="horizontal" style="height:100px" flex="1">
 		<box id="sidebar-box">
 			<splitter class="sidebar-main-header" collapse="after" persist="state">
-				<text class="panel-bar" value="&mail-sidebar-header.label;"/>
+				<text value="&mail-sidebar-header.label;"/>
 			</splitter>
 			<tree id="folderTree" style="width:100px; height: 600px;" flex="1" context="folderPaneContext"/>
 		</box>