i2c/chips: Remove deprecated pcf8574 driver

The pcf8574 driver in drivers/i2c/chips which just exports its register to
sysfs is superseded by drivers/gpio/pcf857x.c which properly uses the gpiolib.
As this driver has been deprecated for more than a year, finally remove it.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>