kbuild: doc: remove the description about shipped files

The use of shipped files is discouraged in the upstream kernel these
days. [1]

Downstream Makefiles have the freedom to use shipped files or other
options to handle binaries, but this should not be advertised in the
upstream document.

[1]: https://lore.kernel.org/all/CAHk-=wgSEi_ZrHdqr=20xv+d6dr5G895CbOAi8ok+7-CQUN=fQ@mail.gmail.com/



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