kbuild: remove unneeded link_multi_deps

Since commit c8589d1e ("kbuild: handle multi-objs dependency
appropriately"), $^ really represents all the prerequisite of the
composite object being built.

Hence, $(filter %.o,$^) contains all the objects to link together,
which is much simpler than link_multi_deps calculation.

Please note $(filter-out FORCE,$^) does not work here. When a single
object module is turned into a multi object module, $^ will contain
header files that were previously included for building the single
object, and recorded in the .*.cmd file. To filter out such headers,
$(filter %.o,$^) should be used here.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>