mfd: ab8500-gpadc: Remove unused 'struct ab8500_gpadc'

The ab8500_gpadc structure carries lots of important information which
most functions make good use of. The initial expectation was that
ab8500_gpadc_runtime_idle() would be no exception; however, this hasn't
been the case to date. Let's remove it for now and add it back in only
when we have a use for it.

Signed-off-by: Lee Jones <lee.jones@linaro.org>