The type of node is called processing-instruction so use
<xsl:template match="processing-instruction()[starts-with(name(), 'mso-')]"/>
together with the identity transformation template
<xsl:template match="@* | node()">
<xsl:copy>
<xsl:apply-templates select="@* | node()"/>
</xsl:copy>
</xsl:template>
That way the processing instructions where the name starts with mso-
will not be copied.
I will show a complete stylesheet:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="text() | comment()">
<xsl:copy/>
</xsl:template>
<xsl:template match="processing-instruction()[starts-with(name(), 'mso-')]"/>
<xsl:template match="*">
<xsl:element name="{local-name()}">
<xsl:apply-templates select="@*|node()"/>
</xsl:element>
</xsl:template>
<xsl:template match="@*">
<xsl:attribute name="{local-name()}">
<xsl:value-of select="."/>
</xsl:attribute>
</xsl:template>
</xsl:stylesheet>