irq: unexport handle_irq_desc()

There are no modular users of handle_irq_desc(). Remove the export
before we gain any.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Suggested-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>