Rework custom man page template from systemd

- remove the non-reproducible top.comment section from the generated man pages
- do not change the other aspects of man page contents

man/custom-man.xsl

@@ -0,0 +1,16 @@
+<?xml version='1.0'?> <!--*-nxml-*-->
+
+<!--
+  SPDX-License-Identifier: LGPL-2.1-or-later
+-->
+
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
+                xmlns:exsl="http://exslt.org/common"
+                extension-element-prefixes="exsl"
+                version="1.0">
+
+<xsl:import href="http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"/>
+
+<xsl:template name="top.comment" />
+
+</xsl:stylesheet>