mfd: arizona: Remove totally unused forward declaration

This declaration has never been used and is likely some left over from
early prototypes of the code, just remove it.
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/arizona.h

@@ -17,8 +17,6 @@
 #include <linux/regmap.h>
 #include <linux/pm.h>
 
-struct wm_arizona;
-
 extern const struct regmap_config wm5102_i2c_regmap;
 extern const struct regmap_config wm5102_spi_regmap;