Require provider api_key and remove env var fallback

examples/read_only/providers.tf

@@ -12,7 +12,6 @@ terraform {
 # This source address stays "dynu/dynu" while using ~/.terraformrc dev_overrides.
 # Terraform will load your local terraform-provider-dynu binary instead of the public registry.
 provider "dynu" {
-  # Use DYNU_API_KEY environment variable by default.
-  # Set var.dynu_api_key in terraform.tfvars to override.
+  # Set var.dynu_api_key in terraform.tfvars.
   api_key = var.dynu_api_key
 }