#1523 make sfml.cbp template a valid XML file

[Dan Horak]

The XML standard doesn't allow the use of double hyphen strings within comments, see https://www.w3.org/TR/REC-xml/#sec-comments for official wording. Thus drop the offending comments as they are not useful anyway (AFAICT).

[Miguel Gimenez]

Applied in [r13649], thank you.