Remove the need for a DATABASE_URL

Since the application does not use a database, it's confusing to have to
set a bogus database URL environment variable. This change follows [the
Lucky guide][1] suggestion for disabling the need for database
configuration. That makes the setup a little easier.

[1]:
https://www.luckyframework.org/guides/database/intro-to-avram-and-orms

src/app.cr

@@ -6,7 +6,6 @@ Lucky::AssetHelpers.load_manifest "public/mix-manifest.json"
 require "./version"
 require "../config/server"
 require "../config/**"
-require "./app_database"
 require "./constants"
 require "./models/base_model"
 require "./models/mixins/**"