docs/website: remove broken symlink for top-level manual

Recently, we've tried to have the manual lie at the top-level of the
site, to get a shorter URL.

However, that is broken: we do not need only a link the the HTML, but
also to the CSS and to any resource linked from the html (images...).

The server configuration has now been updated to add a redirect, so we
no longer need that symlink anyway. Remove it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>