timekeeping: Fold in timekeeping_delta_to_ns()

timekeeping_delta_to_ns() is now called only from
timekeeping_cycles_to_ns(), and it is not useful otherwise.

Simplify the code by folding it into timekeeping_cycles_to_ns().

No functional change.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-16-adrian.hunter@intel.com