kbuild: remove unnecessary "obj- := dummy.o" trick

In these Makefiles, at least one of "obj-y" and "obj-" is non-empty,
hence built-in.o is always created without such a trick.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Peter Foley <pefoley2@pefoley.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Acked-by: Simon Horman <horms+renesas@verge.net.au> [shmobile]
Acked-by: David S. Miller <davem@davemloft.net> [networking]
Signed-off-by: Michal Marek <mmarek@suse.cz>