x86/tsc: Remove obsolete ART to TSC conversion functions

convert_art_to_tsc() and convert_art_ns_to_tsc() interfaces are no
longer required. The conversion is now handled by the core code.

Signed-off-by: Lakshmi Sowjanya D <lakshmi.sowjanya.d@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240513103813.5666-9-lakshmi.sowjanya.d@intel.com