ARM: ixp4xx: remove unused header file pata_ixp4xx_cf.h

Commit b00ced38 ("ARM: ixp4xx: Delete Avila boardfiles") removed the
last use of <linux/platform_data/pata_ixp4xx_cf.h> but left the header file
in place.  Nothing uses this file, delete it now.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>