While I prefer the chosen answer by jnhghy - Jantea Alexandru, the following is an alternative way to do it if you just need something quick:
Change the following code:
$pdf->SetHeaderData(PDF_HEADER_LOGO, PDF_HEADER_LOGO_WIDTH, PDF_HEADER_TITLE.' 058', PDF_HEADER_STRING);
to
$pdf->SetHeaderData(PDF_HEADER_LOGO, PDF_HEADER_LOGO_WIDTH, PDF_HEADER_TITLE.' 058', PDF_HEADER_STRING, array(0,0,0), array(255,255,255));
This simply sets the color of the line to white (the background color of the page). The first array of colors is for the header text color, the second array of colors is for the header line color.