boards: up_squared: kconfig: Remove unused board variant symbols
boards: up_squared: kconfig: Remove unused board variant symbols

The CONFIG_UP_SQUARED_{ATOM,CELERON,PENTIUM} symbols are unused after
commit c5e582038c ("boards/x86/up_squared: default to new local APIC
timer").

Since these symbols are the only thing in boards/x86/up_squared/Kconfig,
which is osource'd in in board/Kconfig, just remove the entire file.

Found with a script.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

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