x86/hpet: Remove unused FSEC_PER_NSEC define

The FSEC_PER_NSEC macro has had zero users since commit

  ab0e08f1

 ("x86: hpet: Cleanup the clockevents init and register code").

Remove it.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20181130211450.5200-1-roland@purestorage.com