ARM: exynos_defconfig: Remove CONFIG_EXYNOS_THERMAL_CORE define

After Exynos TMU rework to use device tree for configuration
this flag can be removed. It is not used anymore.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>