Remove generation date from docs.

The improves the reproducibilit of the build. The date
was nice to have, but there's a version string at the
top which should identify the release correctly.

The copyright date could actually be incorrect, since
it is based on the build date and not the last-changed
date.

doc/footer.html

 <!--BEGIN GENERATE_TREEVIEW-->
-    <li class="footer">$generatedby
+    <li class="footer">Generated by
     <a href="https://www.stack.nl/~dimitri/doxygen/">
     <img class="footer" src="doxygen.png" alt="doxygen"/></a> $doxygenversion </li>
    </ul>
@@ -14,7 +14,7 @@
 For more information visit the <a href="https://opus-codec.org">Opus Website</a>.
       </td>
       <td>
-        <address class="footer"><small>&copy;$year<br/>
+        <address class="footer"><small>
           Generated by
           <a href="https://www.stack.nl/~dimitri/doxygen/">doxygen</a>
           $doxygenversion