mfd: ti_ssp: Remove unused header

The header file include/linux/mfd/ti_ssp.h does not seem to be used
anywhere. It was orphaned by 3033ee62 "mfd: Remove obsolete ti-ssp
driver". Remove it.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>