thermal: Kconfig: Remove config for not used EXYNOS_THERMAL_CORE

After removing exynos_thermal_common.[c|h] files the CONFIG_EXYNOS_THERMA_CORE
is not needed anymore.
This patch removes this entry from Kconfig.

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>