Documentation: kbuild: Remove obsolete include/asm symlink step

As of commit f7f16b77 ("kbuild: drop
include/asm"), the include/asm symlink is no longer created.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>