x86/tsc: Remove unused "US_SCALE" and "NS_SCALE" leftover macros

Last use of them was removed 13 years ago, when the code was converted
to use CYC2NS_SCALE_FACTOR:

  53d517cd: ("x86: scale cyc_2_nsec according to CPU frequency")

The current TSC code uses the 'struct cyc2ns_data' scaling abstraction,
the old fixed scaling approach is long gone.

This cleanup also removes the 'arbitralrily' typo from the comment,
so win-win. ;-)

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200724114418.629021-3-mingo@kernel.org