fixture setup does not rely on `AR::Base.configurations`.

As you can also configure your database connection using `ENV["DATABASE_URL"]`,
the fixture setup can't reply on the `.configurations` Hash.

As the fixtures are only loaded when ActiveRecord is actually used
(`rails/test_help.rb`) it should be safe to drop the check for an existing configuration.
Merge pull request #10907 from senny/fixtures_and_database_url

fixture setup does not rely on `AR::Base.configurations`.