Remove or clarify `io.buildpacks.base.id` (it's currently unused)

[edmorley]

The spec currently says:

For run images, the platform SHOULD ensure that:

• The image config's Label field has the label io.buildpacks.base.id set to the target ID of the run image.

(https://github.com/buildpacks/spec/blob/platform/0.13/platform.md#target-data)

However:

• This label is currently not used by anything (after its removal from the Buildpack API in #374) - though this isn't clear from the spec.
• The spec doesn't explain what a "target ID" is really meant to represent - ie: as a run image maintainer, how do I decide what value to set for it?

I suggest that we either:

1. Remove it from the spec to avoid confusion (it can always be added back later if we decide to re-add a similar concept to the buildpack API spec)
2. Leave it in the spec, but add a note saying it's currently unused.