make help: remove <pkg>-legal-info

The <pkg>-legal-info target is only a component of the top-level
legal-info target, it is not meant to be used alone.

For example, calling twice 'make busybox-legal-info' produces duplicate
entries in licenses.txt and manifest.csv.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>