boards: x86: gpmrb: Remove SYS_CLOCK_HW_CYCLES_PER_SEC default

Let the default value for SYS_CLOCK_HW_CYCLES_PER_SEC come from the
SoC instead. Furthermore, a default for HPET_TIMER didn't even make
sense since this timer doesn't do anything with the Kconfig value.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>