kbuild: remove redundant quiet_modtag for $(obj-m)

All objects in $(obj-m) are contained in $(real-obj-m) as well.

It is true composite objects are only contained in $(obj-m),
but [M] is hard-coded in quiet_cmd_link_multi-m.

This line is redundant.

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