Here is how to do it via XSLT.
The XSLT is using a so called Identity Transform pattern.
I modified XML and removed bogus namespaces.
Input XML
<?xml version="1.0"?>
<root-element>
<tag-name firstline="some-value">bla-bla</tag-name>
<tag-name secondline="some-value">bla-bla</tag-name>
<tag-name thirdline="some-value">bla-bla</tag-name>
<tag-name firstline="some-value">bla-bla</tag-name>
<tag-name secondline="some-value">bla-bla</tag-name>
</root-element>
XSLT
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" encoding="utf-8" indent="yes" omit-xml-declaration="yes"/>
<xsl:strip-space elements="*"/>
<xsl:template match="@*|node()">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="*[@firstline]"/>
</xsl:stylesheet>
Output XML
<root-element>
<tag-name secondline="some-value">bla-bla</tag-name>
<tag-name thirdline="some-value">bla-bla</tag-name>
<tag-name secondline="some-value">bla-bla</tag-name>
</root-element>