x86/Kconfig: Remove HPET_EMULATE_RTC depends on RTC

The RTC config option was removed in commit f52ef24b

 ("rtc/alpha:
remove legacy rtc driver")

Signed-off-by: Anand K Mistry <amistry@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20210204183205.1.If5c6ded53a00ecad6a02a1e974316291cc0239d1@changeid