[web] Do not try to rmtree symlinks in rmtreecontent()
There is a symlink in data directory (created by generate_static_dir() method)
and trying to rmtree() it will fail with an OSError. So we unlink() it
instead.
cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)