Remove unused tag in the akanda-horizon base template

DHC-1003
That tag let us add new css stylesheets if we may want in the
future, but since we have a dreamhost-theme and since is causing
us problems during the deployment, let remove it.

Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>

Change-Id: Ie3069d5e0d531e4f3b4ff91f8317a7164582b42e