Revert "Reject empty database yamls"

This reverts commit 0979713.

I originally wanted to delete the default config but found out it can
be called anything which means the code would blow up in unexpected
ways.

I thought "cool ill just delete the configs without dbs" and realized
that totally 100% breaks the three-tier config. So I'm reverting this
and the other commit.

railties/lib/rails/application/configuration.rb

@@ -179,7 +179,7 @@ def database_configuration
               values.reverse_merge!(shared)
             end
           end
-          loaded_yaml.reject! { |_, values| !values["database"] }
+          loaded_yaml.delete("default")
           Hash.new(shared).merge(loaded_yaml)
         elsif ENV["DATABASE_URL"]
           # Value from ENV['DATABASE_URL'] is set to default database connection