re-remove tun-ipv6 from mullvad configs

[rozab]

Breaking change

I don't think this should be a breaking change because ipv6 didn't seem to be working with mullvad anyway? I am unsure because the author of #200 didn't seem to have problems, perhaps he had it disabled elsewhere.

Proposed change

Re-removed tun-ipv6 from all openvpn configs. This was previously removed in PR #7 but re-added in #200, breaking the configurations for docker containers with ipv6 enabled. Details in issue #1355.

I am experiencing the same errors people had in that issue, and making this change locally has fixed things for me.

Type of change

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New provider (thank you!)
• [x] Updated provider (thank you!)
• [ ] New feature (which adds functionality to a provider script/repo usage)
• [ ] Breaking change (fix/feature causing existing functionality to break)

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue: relates to haugene/docker-transmission-openvpn#1355
• Link to documentation updated (if done separately): https://...

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• [ ] Documentation added/updated

[pkishino]

I had a quick look at #200 and yeah, seeing now it was just left in because that was in default.. it would be nice to get more people who actually use mullvad to comment on this before merging it in