irqchip: gic: Drop support for gic_arch_extn

Now that the users of gic_arch_extn have been fixed, drop the
"feature" for good. This leads to the removal of some now useless
locking.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>