board: frdm_kl25z: Remove undefined Kconfig symbol from defconfig

The defconfig has CONFIG_SOC_MKL25Z=y, however CONFIG_SOC_MKL25Z doesn't
exist and should have been CONFIG_SOC_MKL25Z4,  However its not
necessary to define this at all since it implied by the board.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>