Simplify the image label specification

In the current image specification, we have `ai.backend.resource.min.*` label fields to specify the minimum/default resources slots required for containers using an image.

Pre-specifying these values when building images is becoming impractical as we have multiple different resource slot realizations, even for a single accelerator plugin, depending on the cluster-wide or resource-group-wide configurations.

Let's remove those labels from validation and checks (ignore existing ones) and just assume the sensible default values as follows if the client does not specify resource slots:

- `COUNT` type resource slots: 1 (or 1.0 if it's fractional)
- `BYTES` type resource slots: 1 GiB (1073741824)
- `UNIQUE` type resource slots: 0 (should always be explicitly specified)
(cc: @hephaex)

JIRA Issue: BA-115