kbuild: remove wrong documentation about mandatory-y

This sentence does not make sense in the section about mandatory-y.

This seems to be a copy-paste mistake of commit fcc8487d ("uapi:
export all headers under uapi directories").

The correct description would be "The convention is to list one
mandatory-y per line ...".

I just removed it instead of fixing it. If such information is needed,
it could be commented in include/asm-generic/Kbuild and
include/uapi/asm-generic/Kbuild.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/makefiles.rst

@@ -1379,9 +1379,6 @@ See subsequent chapter for the syntax of the Kbuild file.
 	in arch/$(ARCH)/include/(uapi/)/asm, Kbuild will automatically generate
 	a wrapper of the asm-generic one.
 
-	The convention is to list one subdir per line and
-	preferably in alphabetic order.
-
 8 Kbuild Variables
 ==================