genirq: Remove irq_chip_regs:: Polarity

The polarity member of struct irq_chip_regs is unused. Remove it along
with its kernel-doc.

Found by https://github.com/jirislaby/clang-struct

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Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240808104118.430670-3-jirislaby@kernel.org