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>

Author: masahir0y

Documentation/kbuild/modules.rst

@@ -521,9 +521,3 @@ build.
 
 		ext2-y := balloc.o bitmap.o dir.o
 		ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
-
-	External modules have traditionally used "grep" to check for
-	specific `CONFIG_` settings directly in .config. This usage is
-	broken. As introduced before, external modules should use
-	kbuild for building and can therefore use the same methods as
-	in-tree modules when testing for `CONFIG_` definitions.