u64_stat: Remove the obsolete fetch_irq() variants.

There are no more users of the obsolete interface
u64_stats_fetch_begin_irq() and u64_stats_fetch_retry_irq().

Remove the obsolete API.

[bigeasy: Split out the bits from a larger patch].

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20230110160738.974085-1-bigeasy@linutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>