In this case, instead of xsl:copy use xsl:element to dynamically create the elements with XPath function local-name(). Like this, the namespace won't be copied to the result tree.
Found here
<xsl:template match="*|@*"> <xsl:element name="{local-name()}"> <xsl:copy-of select="@*"/> <xsl:apply-templates/> </xsl:element></xsl:template>