mfd: Delete ab5500-core.h

Commit 72fb9220 ("mfd/ab5500: delete
AB5500 support") deleted all files that used ab5500-core.h. That file
apparently was simply overlooked. Delete it too.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>