Remove tfvars files from Terraform.gitignore

`tfvars` files should not be gitignore-d as they are used to parameterize a Terraform root for multiple environments, e.g.:

```bash
$ tree terraform/roots/vpc
terraform/roots/vpc
├── env
│   ├── production.tfvars
│   ├── qa.tfvars
│   └── staging.tfvars
└── main.tf
```

There may be a use case where a particular `tfvars` file should be ignored, but there's no naming convention I'm aware of to easily designate such a file.