Remove the s3c2410_gpio_irq2pin() function as it is not being
used in any in kernel driver and the function is probably not
being used anywhere else.

This is also part of the effort to remove any of the s3c24xx gpio
specific code that cannot be recreated by using the gpiolib
framework now in the kernel.
Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>