Remove gitter integration

[jugmac00]

It looks like the tox gitter community was created in 2018. While it had its use in the past, it is no longer used.

Also, meanwhile we have a discord instance as a direct replacement. Additionally, we have discussions on the git repo and we (or at least I) monitor also StackExchange.

Let's make support a bit more focused and close gitter.

There is no way to do it via the gui - having a look at https://gitlab.com/gitterHQ/webapp/blob/develop/docs/communities.md#delete-a-community I think @gaborbernat has to send an email to support@gitter.im

I will create a pull request to remove the link to gitter at https://github.com/tox-dev/tox/issues/new/choose

• remove link from the list when creating a new issue
• delete the gitter instance -> @gaborbernat needs to send an email to support@gitter.im

[gaborbernat]

I did email them 👍

[jugmac00]

The link to check whether it is still up or not:
https://gitter.im/tox-dev/Lobby

[gaborbernat]

Done.