dnsapi: deSEC.io: Remove need for DEDYN_NAME environment variable

[s-hamann]

This PR removes the need for the $DEDYN_NAME environment variable for the deSEC.io DNS provider. Instead, $_domain is used, which is automatically set by _get_root and contains the same information.

[auto-comment]

First, NEVER send a PR to master branch, it will NEVER be accepted. Please send to the dev branch instead.
If this is a PR to support new DNS API or new notification API, please read this guide first: https://github.com/acmesh-official/acme.sh/wiki/DNS-API-Dev-Guide
Please check the guide items one by one.
Then add your usage here: https://github.com/acmesh-official/acme.sh/wiki/dnsapi

Or some other wiki pages:
https://github.com/acmesh-official/acme.sh/wiki/deployhooks
https://github.com/acmesh-official/acme.sh/wiki/notify