core: drop now-useless prepare-kconfig rule

core: drop now-useless prepare-kconfig rule

This rule was added back in 9429e7b698 (core: introduce an intermediate
rule before the configurators) when the kconfig-side br2-external file
was generated separately from the Makefile-side one.

Now that they are generated together very early in the Makefile, we no
longer need this intermediate rule. Drop it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
[Peter: also drop outdated reference in the manual]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>