ARM: exynos4_defconfig: remove obsolete exynos4_defconfig

It hasn't been updated since Novemeber 2011 and exynos_defconfig
should be used instead.

After conversion to using device tree only (no board files) on
mach-exynos using exynos4_defconfig results in unbootable kernel.

There is no much point in trying to fix exynos4_defconfig because
exynos_defconfig also works for EXYNOS4 SoCs, is being kept up to date
(for the comparison the last exynos4_defconfig update was in November
2011) and has more useful stuff enabled (i.e. exynos4_defconfig doesn't
even have EXT3 or EXT4 enabled, it only has EXT2 turned on).

Another advantage of using one defconfig for both EXYNOS4 and EXYNOS5
is that it makes development and testing easier. It should allow us
to catch early EXYNOS4 regressions caused by EXYNOS5 changes and also
EXYNOS4 changes that break EXYNOS5.

Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>