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

This reverts commit 84c825f8e8.

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>

docs/manual/customize-outside-br.txt

@@ -107,17 +107,3 @@ And then in +$(BR2_EXTERNAL)/package/package1+ and
    output of +make list-defconfigs+ and allow them to be loaded with the
    normal +make <name>_defconfig+ command. They will be visible under the
    +User-provided configs+' label in the 'make list-defconfigs' output.
-
-Additionally, an +external.mk+ file may define the +help-custom+ make
-rule, to document custom make targets specific to this +BR2_EXTERNAL+
-tree. The help is completely free-form.
-
-------
-help-custom:
-    @echo 'Here goes your local help, where you may'
-    @echo 'describe some custom rules:'
-    @echo '  my-rule       - do something'
-    @echo '  my-other-rule - do something else'
-    @echo
-    @echo 'Please contact support@company.com in case of problem.'
-------