Update ejabberd to not use Distillery

[badlop]

Support to build OTP relases using Elixir was first added to ejabberd in commit 4ae7c81 for 15.09 using the libraries exrm and relx:

exrm, or Elixir Release Manager, provides mix tasks for building, upgrading, and controlling release packages for your application.

relx is a library that assembles Erlang/OTP releases. Given a release specification and a list of directories in which to search for OTP applications it will generate a release output. It is generally used through the Erlang/OTP build tool rebar3 which provides a cli interface.

Those libraries were replaced with Distillery to ejabberd in commit 1c8edd0 for release 16.12, and in docker-ejabberd in commit 9d97b7d for 19.08:

Distillery: Build releases of your Mix projects with ease!

But nowadays, Elixir doesn't need Distillery to build OTP releases... Release support, with a different implementation, was included in Elixir v1.9, check the Mix Release documentation.

Tasks:

• Don't include Distillery dependency in ejabberd: done in a846260
• Don't use Distillery in docker-ejabberd: done in processone/docker-ejabberd@2ae9f00
• Don't use Distillery in ProcessOne binary installers. Check new installers work correctly
• Inform about this in the next release notes, in case some packager still used Distillery