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 <[e-mail]>
Acked-by: Peter Foley <[e-mail]>
Acked-by: Mike Turquette <[e-mail]>
Acked-by: Simon Horman <[e-mail]> [shmobile]
Acked-by: David S. Miller <[e-mail]> [networking]
Signed-off-by: Michal Marek <[e-mail]>

arch/arm/plat-samsung/Makefile

···
 
 obj-y				:=
 obj-m				:=
-obj-n				:= dummy.o
 obj-				:=
 
 # Objects we always build independent of SoC choice

drivers/clk/shmobile/Makefile

···
 obj-$(CONFIG_ARCH_R8A7791)		+= clk-rcar-gen2.o
 obj-$(CONFIG_ARCH_SHMOBILE_MULTI)	+= clk-div6.o
 obj-$(CONFIG_ARCH_SHMOBILE_MULTI)	+= clk-mstp.o
-# for emply built-in.o
-obj-n	:= dummy

drivers/net/wimax/Makefile

···
-
 obj-$(CONFIG_WIMAX_I2400M)	+= i2400m/
-
-# (from Sam Ravnborg) force kbuild to create built-in.o
-obj- := dummy.o