boards: mec172xevb: Fix cmake no sources warning
boards: mec172xevb: Fix cmake no sources warning

Microchip MEC172x EVB no longer has any C files in board
folder. Remove the cmake library rule as it it causing
a build warning due to no source files.

Signed-off-by: Jay Vasanth <f29a0b91d696aa12caa0e00daf52bda6a5259c6c@microchip.com>

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