Remove mailgun from devcentral maillig system

Summary:
The local delivery of mails for devecentral is working flowlessly
since the summer. The plan now is to remove mailgun as secondary
to remove the service and the fee linked to it.

Ref T2100

Test Plan: remove the configuration, deploy it and cancell the mailgun service

Reviewers: dereckson, Duranzed

Reviewed By: Duranzed

Maniphest Tasks: T2100

Differential Revision: https://devcentral.nasqueron.org/D3949