cmake: kconfig: Remove unused extra dtc flags
cmake: kconfig: Remove unused extra dtc flags

The usage of EXTRA_DTC_FLAGS was removed in commit
269f35048770cc981a1db1ecc770ded62e141291

If ninja was used and e.g. menuconfig is called, when more than one
element is in EXTRA_DTC_FLAGS, calling "ninja menuconfig" failed.

E.g.
west build -b lpcxpresso55s28_ns -t menuconfig samples/hello_world
resulted in and error. This is fixed with this commit.

Signed-off-by: Christian Taedcke <5dfb12f5b8690cb9c18e955016e7e3173b81d0d8@lemonbeat.com>

galak/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr