base: image-license-checker: drop bbclass

We can drop this bbclass since we can cover most of the existing cases with
the existing upstream variables INCOMPATIBLE_LICENSE and INCOMPATIBLE_LICENSE_EXCEPTIONS
plus our recently added IMAGE_INCOMPATIBLE_LICENSE and IMAGE_INCOMPATIBLE_LICENSE_EXCEPTIONS.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>

Author: quaresmajose