Revert "core: add the possibility to provide help for custom rules"

Revert "core: add the possibility to provide help for custom rules"

This reverts commit 84c825f8e893bfb56847ab4a880c46066a41744f.

Turns out that the custom help is not available when the $(O) directory
has not been configure yet (i.e. when there is no .config already
filled).

Rather than trying to work around this limitation with dirty hacks, just
revert this feature. After all, this will not prevent an external.mk
from providing custom help anyway; it's just not gonna be advertised nor
displayed with the main help.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>