Remove the need for multiple config files

This commit removes the almost a 100% redundant tests (two fields were
checked differently) and makes a single example configuration for users.

config.yaml.sqlite.example

@@ -1,26 +0,0 @@
----
-server_url: http://127.0.0.1:8080
-listen_addr: 0.0.0.0:8080
-private_key_path: private.key
-derp_map_path: derp.yaml
-ephemeral_node_inactivity_timeout: 30m
-
-# SQLite config (uncomment it if you want to use SQLite)
-db_type: sqlite3
-db_path: db.sqlite
-
-acme_url: https://acme-v02.api.letsencrypt.org/directory
-acme_email: ''
-tls_letsencrypt_hostname: ''
-tls_letsencrypt_listen: ":http"
-tls_letsencrypt_cache_dir: ".cache"
-tls_letsencrypt_challenge_type: HTTP-01
-tls_cert_path: ''
-tls_key_path: ''
-acl_policy_path: ''
-dns_config:
-  nameservers:
-  - 1.1.1.1
-  domains: []
-  magic_dns: true
-  base_domain: example.com