Remove temporary entry for squirrelmail

The issues in from https://sourceforge.net/p/squirrelmail/bugs/2831/ as
reported by Hanno Böck did already got assigned

	CVE-2018-14950: SquirrelMail through 1.4.22 has XSS via a "<svg><a xlink:href=" attack.
	CVE-2018-14951: SquirrelMail through 1.4.22 has XSS via a "<form action='data:text" attack.
	CVE-2018-14952: SquirrelMail through 1.4.22 has XSS via a "<math><maction xlink:href=" attack.
	CVE-2018-14953: SquirrelMail through 1.4.22 has XSS via a "<math xlink:href=" attack.
	CVE-2018-14954: The mail message display page in SquirrelMail through 1.4.22 has XSS via the formaction attribute.
	CVE-2018-14955: SquirrelMail through 1.4.22 has XSS via SVG animations (animate to attribute).