Remove requirement of order.toml from builder

[jromero]

I suggest that we remove this requirement entirely given that we haven't thought about how to handle the case where we change or remove this file between platform APIs. The platform can always pass in a valid order.toml for it's preferred platform API, created from the label if it wants to use the builder-provided default.

Originally posted by @ekcasey in https://github.com/buildpacks/spec/pull/271#discussion_r787977057