Remove optional postfix dependency

[evilhamsterman]

@geerlingguy This is the same idea as #86 but simpler, there's no need to make the dependencies a variable because there are just ones listed by Gitlab as necessary. But as stated in #86 postfix is optional and your role doesn't configure it. Since Gitlab can use direct SMTP or another sendmail option that should just simply be left to the users playbook to configure. If the need other dependencies then those should be other roles or part of the playbook.

I'm in the same situation as the guy in #86 we use nullmailer and have a role defined which sets it up and this role conflicts right now.

[stale]

This pull request has been marked 'stale' due to lack of recent activity. If there is no further activity, the PR will be closed in another 30 days. Thank you for your contribution!

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[eRadical]

This should be still active.

[stale]

This issue is no longer marked for closure.

[geerlingguy]

It is now possible (after dfdfa1f) to override the gitlab_dependencies list and swap postfix for exim (or remove it and depend on something else entirely).