kbuild: doc: remove description about grepping CONFIG options

This description was added 20 years ago [1]. It does not convey any
useful information except for a feeling of nostalgia.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=65e433436b5794ae056d22ddba60fe9194bba007



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>