As others have pointed out, you may not want to do this. If you want the output to be XHTML, you need to keep the XHTML namespace declaration.
That being said, if you really want to do it:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<!-- attributes, commments, processing instructions, text: copy as is -->
<xsl:template match="@*|comment()|processing-instruction()|text()">
<xsl:copy-of select="."/>
</xsl:template>
<!-- elements: create a new element with the same name, but no namespace -->
<xsl:template match="*">
<xsl:element name="{local-name()}">
<xsl:apply-templates select="@*|node()"/>
</xsl:element>
</xsl:template>
</xsl:stylesheet>