genirq: Remove redundant NULL pointer check in __free_irq()

The NULL pointer check in __free_irq() triggers a 'dereference before NULL
pointer check' warning in static code analysis. It turns out that the check
is redundant because all callers have a NULL pointer check already.

Remove it.

Signed-off-by: RAGHU Halharvi <raghuhack78@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20180717102009.7708-1-raghuhack78@gmail.com