Require provider api_key and remove env var fallback

examples/read_only/terraform.tfvars.example

@@ -1,6 +1,5 @@
 # Copy to terraform.tfvars and adjust values for your environment.
-# dynu_api_key can be omitted when DYNU_API_KEY is exported in your shell.
-# dynu_api_key = "replace-with-dynu-api-key"
+dynu_api_key = "replace-with-dynu-api-key"
 
 # Use a hostname that exists in your Dynu account.
 hostname = "www.example.com"