kbuild: remove unnecessary variable initializaions

Clearing obj-y, obj-m, obj-n, obj- in each Makefile is
a useless habit.

They are non-exported variables; therefore they are always empty
whenever descending into each subdirectory.
(Moreorver, obj-y and obj-m are also set to empty at the beginning
of scripts/Makefile.build)

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Peter Foley <pefoley2@pefoley.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>