genirq/debugfs: Remove redundant NULL pointer check

debugfs_remove() can be called with a NULL pointer.

Fixes: 087cdfb6 ("genirq/debugfs: Add proper debugfs interface")
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>