ARM: 8580/1: Remove orphaned __addr_ok() definition

Since commit 8c56cc8b ("ARM: 7449/1: use generic strnlen_user and
strncpy_from_user functions"), the definition of __addr_ok() has been
languishing unused; eradicate the sucker.

Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>