web: Remove SSI for last modification in footer.html.

web: Remove SSI for last modification in footer.html.

The footer.html file is is only used for the doxygen pages. It contains
a server side include directive although SSI is not activated on the
people.tuebingen.mpg.de web server and probably will never be.

This commit gets rid of the SSI and also removes the end tags </td>
and </table> from footer.html as these elements are not open at this
point of the document. This was found by the w3c validator.

Removes also the link to the W2C validator. Nobody wants to see this,
and doxygen does not even create valid html anyway..