boards: up_squared: kconfig: Remove unused board variant symbols

The CONFIG_UP_SQUARED_{ATOM,CELERON,PENTIUM} symbols are unused after
commit c5e58203 ("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 <Ulf.Magnusson@nordicsemi.no>