luci-app-acme: remove handling of deprecated `state_dir` option

Recent versions of "acme" report the following message on startup:

    daemon.warn acme: Option "state_dir" is deprecated, please remove it.
                      Certificates now exist in /etc/ssl/acme

Solve this issue by removing the support for this deprecated option.

Fixes: #6273
Signed-off-by: Jo-Philipp Wich <jo@mein.io>