x86/tsc: Move away from TSC leaf magic numbers

The TSC code has a bunch of hard-coded references to leaf 0x15.  Change
them over to the symbolic name.  Also zap the 'ART_CPUID_LEAF' definition.
It was a duplicate of 'CPUID_TSC_LEAF'.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20241213205034.B79D6224%40davehans-spike.ostc.intel.com