kbuild: remove $(MODLIB)/source symlink

This reverts the old commit "kbuild: Introduce source symlink in
/lib/modules/.../". [1]

The current Kbuild does not require $(MODLIB)/source. If the kernel was
built in a separate output directory, $(MODLIB)/build/Makefile wraps
the Makefile in the source tree. It is enough for building external
modules.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=e09e58867154b8aae0a3ac26a9b1c05962f5a355

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>