Clarify in models README's that the IDs in the "model's outputs" are not the real IDs (or, better, remove them)

In the models' README, we have a table with the list of possible outputs and IDs. These IDs are NOT the IDs / indexes of the model's softmax layer (undefined is not a real model output, but it is a possible value for `dl.output`). We should probably clarify this nit, as it may lead to false assumptions. The real source of truth for ID <=> content type is the model's config target labels.

Update: we should likely just remove them. /cc @ia0

Open the models README and inspect the table listing possible outputs and IDs. Remove the IDs, or otherwise make clear that they are not softmax-layer IDs and that config target labels are the source of truth. Done means the README no longer invites false assumptions about output IDs.

Written by the indexing model from the issue text.