Bugfix[#1259]: remove mail nickname check

[DeviaVir]

We are already always setting a default mail nickname, the check seems to fall apart for not-yet-existing groups, but the check itself doesn't make sense since we are just setting a default if none exists.

[manicminer]

@DeviaVir Thanks for suggesting this change. Unfortunately, mailNickname is a required property for mail-enabled groups as per validation in the API. When creating a security-only group, we mimic the portal behavior by generating a pseudorandom value for mailNickname, but since this property is immutable once set, we don't do the same for mail-enabled groups as this would likely defeat the intent of the group.

If you are managing existing groups with Terraform, you will need to import the group into your statefile, and then ensure the correct properties are set in your configuration. This will include the mail_nickname property.

I appreciate the time taken to test and submit this PR, however due to the above we will unfortunately not be able to relax the validation here as it does mirror the API. If you would like to manage groups without managing the mail_nickname property, I recommend looking at the ignore_changes lifecycle meta-argument.

Accordingly, I'm going to close this PR for now. Thanks again!